"The accumulation of all powers, legislative, executive, and judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, selfappointed, or elective, may justly be pronounced the very definition of tyranny."I ask my readers to pause for a moment and ponder this statement. It was penned by James Madison in 1788, published under the pseudonym "Publius" as part of Federalist Paper no. 47. Mr. Madison was an important framer of the United States constitution; along with Alexander Hamilton and John Jay he wrote the Federalist Papers to convince the American public that the constitution they helped create should be ratified as the Union's founding charter. This particular essay was directed against "the more respectable adversaries of this Constitution" who believed that it "[violated] the political maxim, that the legislative, executive, and judiciary departments ought to be separate and distinct." Were this claim true and "the federal Constitution, therefore, really chargeable with the accumulation of power, or with a mixture of powers, having a dangerous tendency to such an accumulation, no further arguments would be necessary to inspire a universal reprobation of the system."
Consider what James Madison conceded in the introduction to his argument. If the form of government established by the constitution did not provide for the separation of powers, no other objections to the constitution would be needed. If the only flaw his adversaries could find in the constitution was that one of its three branches had "a dangerous tendency" to usurp power from its sisters, then every single argument proposed in the 46 preceding essays (to say nothing of the 40 that would follow) would mean nothing. If this charge could not be met then the new federal government would be the definition of tyranny.
The constitutional balance so carefully framed by the members of the Constitutional Convention, defended by Mr. Madison so vigorously afterwords, and subsequently ratified by the 13 independent states of America is dead. The light of legislative branch waned as the 20th century progressed; the 21st century has seen it snuffed out completely.
The total eclipse of the legislative branch is seen clearly in the realm of foreign policy. Jim Webb, Senator (D-VA) from 2007 to 2013, provides a sobbering inside account of how defunct the relationship between the executive and legislative branches has become:
"On the issues of who should decide when and where to use force and for how long, and what our country’s long-term relations should consist of in the aftermath, Congress is mostly tolerated and frequently ignored. The few exceptions come when certain members are adamant in their determination to stop something from happening, but even then they do not truly participate in the shaping of policy.This is seen with the Bush Administration's approach to Iraq:
In December 2008, after more than a year of largely secret negotiations with the Iraqi government, the outgoing George W. Bush administration signed an ambitious, far-reaching document called the Strategic Framework Agreement (SFA). Not to be confused with the mundanely technical Status of Forces Agreement, a common document that with minor variations governs jurisdiction over U.S. forces serving in nearly ninety countries around the world, the SFA addressed a broad range of issues designed to shape the future relationship between the United States and Iraq. This was not quite a treaty, which would have required debate on the Senate floor and the approval of sixty-seven senators, but neither was it a typical executive-branch negotiation designed to implement current policy and law. Included in the SFA, as summarized in a 2008 document published by the Council on Foreign Relations, were provisions outlining “the U.S. role in defending Iraq from internal and external threats; U.S. support of political reconciliation; and U.S. efforts to confront terrorist groups,” as well as measures “shaping future cooperation on cultural, energy, economic, environmental, and other issues of mutual interest.”
Despite years of combat in Iraq, the expenditure of hundreds of billions of dollars of national treasure and deep divisions that remained in the American body politic regarding our future role in this tumultuous region, over the period of more than a year during which the Iraqi SFA was negotiated and finalized, Congress was not consulted in any meaningful way. Once the document was finalized, Congress was not given an opportunity to debate the merits of the agreement, which was specifically designed to shape the structure of our long-term relations in Iraq. Nor, importantly, did the congressional leadership even ask to do so.
Until finalized, the agreement was kept from public and media scrutiny, to minimize any debate that might have put it into jeopardy. From the overt and palpable body language of the executive branch, it was clear that opening up such an important and time-sensitive issue for congressional or public scrutiny would be counterproductive. When this writer asked to read the full document in the weeks before it was signed, I was required to do so inside a soundproof room normally reserved for reviewing classified materials, even though the proposed agreement was not itself classified. And from the logbook I signed before being able to read (but not copy or take with me) the agreement, it appears that I was the only member of the Senate who at least at that point had actually read it.
Congress did not debate or vote on this agreement, which set U.S. policy toward an unstable regime in an unstable region of the world. By contrast, the Iraqi parliament voted on it twice.
Mr. Web describes a similar story behind the creation of Obama administration's policy toward Afghanistan (included is the delightfully cutting observation "the Obama administration has proven itself to be acutely fond of executive orders designed to circumvent the legislative process") before turning to a more recent example of executive usurpation:
THE FAILURE of Congress to meet its historical obligations while the president unilaterally engaged in combat operations in Libya promises even deeper consequences for future crises. In many international situations the future promises a different kind of warfare, made possible (and politically more complex) by the use of special-operations forces, CIA operatives, drones and precision munitions, thus removing the average American from the consequences and even the direct knowledge of military actions that a president might undertake at his or her sole discretion. But to what extent should this “cleaner” way of war also remove Congress as an arbiter of when and where our nation should become involved in overseas hostilities?I recommend reading the whole thing. Former Senator Webb correctly notes that the SFA with Iraq and a similar arrangement with Afghanistan did more than just tip the scales of separated powers towards the executive - they were creations entirely independent of the framework set forth by the United States Constitution.
....The issue in play in Libya was not simply whether the president should ask Congress for a declaration of war. Nor was it wholly about whether Obama violated the edicts of the War Powers Act, which in this writer’s view he clearly did. The issue that remains to be resolved is whether a president can unilaterally begin, and continue, a military campaign for reasons that he alone defines as meeting the demanding standards of a vital national interest worthy of risking American lives and expending billions of dollars of taxpayer money.
....In a world filled with cruelty, the question is not only how but whether a president should be allowed to pick and choose when and where to use military force on the basis of such a vague standard. Given our system of government, the fundamental question is: Who should decide? And even if a president should decide unilaterally on the basis of an overwhelming, vital national interest that requires immediate action, how long should that decision be honored, and to what lengths should our military go, before the matter comes under the proper scrutiny—and boundaries—of Congress?
As a measure for evaluating future crises, it is useful to review the bidding that led to our actions in Libya. What did it look like when President Obama ordered our military into action in that country, and what has happened since?
Was our country under attack, or under the threat of imminent attack? No. Was a clearly vital national interest at stake? No. Were we invoking the inherent right of self-defense as outlined in the UN Charter? No. Were we called upon by treaty commitments to come to the aid of an ally? No. Were we responding in kind to an attack on our forces elsewhere, as we did in the 1986 raids in Libya after American soldiers had been killed in a Berlin disco? No. Were we rescuing Americans in distress, as we did in Grenada in 1983? No.
The president followed no clear historical standard when he unilaterally decided to use force in Libya. Once this action continued beyond his original definition of “days, not weeks,” into months and months, he did not seek the approval of Congress to continue military activities. And, while administration members may have discussed this matter with some members of Congress, the administration never formally conferred with the legislative branch as a coequal partner in our constitutional system.  (emphasis added)
Just as sobering is Mr. Web's description of the new type of war waged by American forces across Africa and the Middle East, typified by American military action in Libya. This type of conflict is fundamentally different from past American military campaigns. Writing for the naval affairs blog Information Dissemination, "Galrahn" comments on the revolutionary nature of these operations:
"But that's the key issue - our small special forces teams are supported by a US conventional military infrastructure of precision strike capabilities that outmatches everything else in the world. Some of our smallest special operations forces have the firepower and capability equivalent to the entire military of some nations at their fingertip. There is nothing new about special forces, but there is a lot of new about featuring special forces operations as a primary instrument of State power. Do the old rules that govern use of military force apply sufficiently to global precision strike capabilities of platoon sized forces with brigade level firepower at their fingertips? Do we need new rule sets for the emerging predominance of special forces operations in our global military taskings? Will America have to wait for Murphy's Law to kick us between the legs before Congress decides to rethink how the emerging predominance of special forces operations influences existing laws regarding the use of military force globally? When these guys come home from Afghanistan, they are not going to be sent home to sit on the couch - they will be used, everywhere else.
Has Congress sufficiently thought through this? With the nations mature long range strike network from air and sea, a special forces unit today can leverage the firepower of at least a battalion sized unit 20 years ago, but we never sent units with battalion levels of firepower around the world on quick strike missions 20 years ago. Emerging policy is to send special forces around the world on quick strike missions for the next 20 years, and my gut tells me our political leaders haven't stepped through this mentally yet."  (emphasis added)
Galrahn calls for the "House and Senate to step up and wake up" and provide much needed oversight for these campaigns. This is harder than it sounds. Even without legislative malaise or executive malfeasance, the shift from leveraging large army contingents (as done by President Bush and co.) to the relying on drones and special forces (as practiced by the present administration) impedes legislative control and oversight. There are very practical reasons for this. Gregory McNiel describes the hoops Senators and Congressmen must jump through in order to provide the sort of oversight Galrahn is pushing for:
[Consider] the Senate Select Committee on Intelligence oversees intelligence activities. If in the conduct of its oversight it was concerned about a particular targeted killing practice, such as the criteria used to add someone to a kill-list, the committee could threaten to cut off funds until more information was provided about the kill-list criteria. However, it would not be much of a threat because appropriations are handled by the Senate Defense Appropriations Subcommittee, and there is little membership overlap between the two committees. This means that members of the Intelligence Committee overseeing targeted killings would need to enlist the support of members of the Appropriations Subcommittee to cut off funds. Here again, secrecy poses a problem because intelligence budgets are classified. Members of Congress serving on either the intelligence committees or the defense appropriations committees (which appropriates funds for intelligence activities) can access the budget, but even their security-cleared staff and other members cannot.
Imagine how this would play out in practice. Intelligence Committee members themselves would need to contact just the appropriators (not their staff). When contacting them, the Intelligence Committee members would not be able to disclose to the appropriators any details about classified activities other than the general line items in the budget that relate to those activities. Thus, the intelligence overseers would need to convince other members to cut off funds based on generalized concerns, rather than any specific details. And all of this assumes that the members have the time and inclination to spend on fighting these fights. Given these facts, when it comes time to threaten to cut off funding for some executive branch malfeasance, it is not surprising that the executive branch sees these threats as hollow and may choose to delay or even ignore a congressional request. While the threat may exist, in reality it is an idle one as only a handful of members will be able to find out the information necessary to make a credible threat, they will not be able to share that information publicly, and they will not be able to share it with other members to build broader congressional support for withholding funds associated with the inappropriate activity. In short, diffused authority combined with secrecy may allow the executive branch to dodge accountability. 
In sum: The power of the legislature has been declining for decades. When C. Wright Mills wrote The Power Elite in 1956 he bemoaned the eclipse of that branch by the influence of corporate power and an ever-growing executive branch.  He was correct to do so. Yet in his day no executive would have dared to agree to a treaty that was never presented to the Senate. The last two administrations have done that and more. This is our 21st century reality. Ours is a Republic that allows its executive branch to pursue strategies and wage wars that undermine the other branches of its government, and with them the constitutional checks placed upon it. The separation of powers is dead. We are left to wonder what James Madison would think.
 James Madison. Federalist. No 47. or. pub. 1 February 1788. [link]
 Jim Webb. "Congressional Abdication." National Interest. March/April 2013 Issue.
 Galhrann. 'With Great State Power Comes Greater Need For Oversight Responsibility." Information Dissemination. 7 March 2013.
 Gregory McNiel. "The Politics of Accountability for Targeted Killings." Lawfare Blog. 14 March 2013.
 C. Wright Mills. The Power Elite. (New York: Oxford University Press). 1956. p. 229-231, 248-269. Despite its age, I have not come across a better explanation for the legislature's demise than that found inside this book.