The major difference between Theodore Roosevelt and his more recent predecessors in the White House was that his foreign policy was much more aggressive. This was the era of the “big stick” in which the US asserted its power around the world, and particularly in the Caribbean. However, it is not clear if this is really a change that comes about because of Roosevelt or because of a change in the country’s circumstances.
By 1901 when McKinley was killed and Roosevelt took office, the US was much stronger than it had been in the past. Industrialization had built its capacity to do things like building ships. Alfred Thayer Mahan’s doctrines had been accepted by many Americans and the US Navy was larger and more powerful than in the past. The US had been getting more aggressive, as seen in things such as the the annexation of Hawaii and the Spanish-American War.
This meshed perfectly with Roosevelt’s own inclinations. He was inclined to follow a foreign policy that explicitly sought to increase American power. He was more aggressive in pursuing American power around the globe than previous presidents had been.