Theodore Roosevelt later said of his presidency that while he didn't seize power, he did stretch executive authority beyond its previous scope [source: Bill of Rights Institute]. On the home front, for instance, he intervened personally to settle labor disputes, pressed hard to regulate (and break) corporate monopolies and exercised his authority to conserve vast tracts of wilderness in the American West.
He was also extremely active on the foreign stage, becoming the first American to receive a Nobel Peace Prize for his negotiations to end the Russo-Japanese War of 1904-1905. And his initiative to buy a section of Panama and dig a big ditch through it laid the groundwork for America's emergence as a world power in the 20th century. It wasn't that the Constitution explicitly forbade presidents from doing such things, it's just that it didn't explicitly say they could. As far as Teddy Roosevelt was concerned, absence of prohibition was an invitation to action. He had a number of things he wanted to accomplish and felt that the executive branch was the venue in which to accomplish them.
Since Roosevelt, successive presidents have sought to expand the reach of their power. Take that "technicality" mentioned earlier — the requirement that a president receive approval from Congress before taking the country to war. This protocol was respected through both world wars, but the Cold War created special conditions. In the 1950s, fear of the Soviet threat was such that presidents could skirt Congress in the interests of national defense. So, for example, Harry Truman was able to able to engage U.S. troops in the Korean conflict without Congressional approval by avoiding war terminology. It was a police action, he said, undertaken jointly with the UN — nothing Congress needed to worry about. It was a precedent that others, including Lyndon Johnson and Bill Clinton, would follow.
By the time Dwight Eisenhower took office, the president had a shady new tool to use — the Central Intelligence Agency. With clandestine operations fully in play, constitutional niceties were unofficially off the table. When John F. Kennedy took office in 1960 it was generally understood that the president, and not the Senate Foreign Relations Committee, was now the big boss of U.S. international affairs.
But great power confers a special vulnerability. Harry Truman famously said, "The buck stops here" — and that's just it. If a president who wants to wield enormous authority goes out on a limb, that limb bends with the weight of popular opinion. When the Korean War went south, Truman was held responsible (since he was), and his approval numbers went south with it.
Understanding the need for popular support, Eisenhower extracted the U.S. from Korea and used diplomacy, brinkmanship and covert operations to successfully contain the Soviet threat. Thanks to these methods, which required less blood and treasure than outright war, he was able to maintain his popularity, which in turn secured his hold on executive power in the sphere of foreign relations [source: Dallek].