Theodore Roosevelt greatly increased the power of the presidency, largely by taking the position that the president could exercise any right not specifically denied him by the Constitution. Previously, Congress had been the most powerful branch of the government, but Roosevelt's presidency helped establish an influential and authoritative executive branch.
Before Roosevelt, business interests held inordinate power in the government, and the country's biggest employers were generally allowed to do whatever they liked. Roosevelt began regulating businesses, even going so far as to break up trusts that had gained too much market share and become effective monopolies. Roosevelt still recognized the value of large businesses to a healthy economy, but he insisted the public must be protected as well.
Roosevelt also saw an increased role for the president in foreign policy. His often repeated motto "Speak softly and carry a big stick," described his technique of being heavily engaged in foreign affairs in the American sphere of influence but not being afraid to impose America's will when needed.
He also saw the president's role as not only to defend the citizens, but also to defend the very resources of the country. To that end, he established national conservation programs to protect natural resources, placing more than 230 million acres of American land under federal protection.