Although there are many examples of conflicts between branches of government, the most dramatic instances tend to involve overreach--or at least perceived overreach--by the executive branch. Events like the Cuban Missile Crisis unfold very rapidly, which makes a formal discussion among the three branches impossible. Even in cases where there is more time for deliberation, the president is granted constitutional authority not to disclose information that could potentially be useful to the nation's enemies.

As you might imagine, this principle has occasionally been misused. A particularly notorious example is President Richard Nixon's attempt to withhold recorded telephone conversations from Congress. The conversations related to Nixon's reelection campaign and revealed illegal activity, but the president claimed that he was withholding the tapes to protect national security. When the truth was uncovered, Nixon resigned rather than face lengthy Senate impeachment hearings.

President Nixon, with edited transcripts of Nixon White House Tape conversations during broadcast of his address to the Nation.

There are plenty more examples of top-ranked government officials exceeding their constitutional authority to serve personal interests. Most of these attempts seem to backfire rather spectacularly, as was the case during the Tea Dome Scandal of 1921, which involved bribery and the use of state influence to conduct private business deals. President Warren Harding's Secretary of the Interior, Albert Bacon Fall, had leased some oil reserves in Teapot Dome, Wyoming and two other locations to private oil companies at low rates and without seeking bids from other companies. In 1922 and 1923, the leases became the subject of a Senate investigation and Fall was convicted of accepting bribes from the oil companies. Fall became the first Cabinet member in American history to go to prison.

President Trump, joined by Vice President Pence, displays his signed executive order for the Establishment of a Presidential Advisory Commission on Election Integrity.

One of the more obvious and visible conflicts between branches happens when the federal courts manage to overturn an executive order. Constitutionally, the president has unlimited power to introduce emergency measures when national security is at stake, and the Constitution also protects the president from having to disclose his or her reasons for such measures. During World War I, President Wilson did not seek congressional approval when he issued an executive order banning antiwar speech, and the courts allowed the order to be implemented. In a more recent example, though, President Donald J. Trump's efforts to exclude citizens of specified nations on the basis of their potential terrorist affiliations were met with strong opposition from the judicial branch; lower federal courts repeatedly blocked the president's order, suggesting it was unconstitutional. However, the Supreme Court upheld parts of the ban until arguments could be made on both sides. From one point of view, the courts created an obstacle for the executive branch; from another perspective, the situation suggests that the system of checks and balances is functioning as it was intended.

Founding Father James Madison and the 18th century French judge Charles Montesquieu were perhaps the  foremost experts of their day on the principle of separation of powers. In Federalist Paper, No. 48, Madison describes the danger of executive overreach bluntly: "The accumulation of all powers legislative, executive, and judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, self-appointed, or elective, may justly be pronounced the very definition of tyranny." It is not a coincidence that Montesquieu's Spirit of the Laws also emphasizes the term tyranny in its warning about the merging of legislative and executive authority. In 1748, Montesquieu wrote that if these two powers are "united in the same person or body, there can be no liberty."