When Theodore Roosevelt succeeded William McKinley as president in 1901, he was aware that America was in a different international position than it had been just a few years earlier. The United States had been a continental empire since its founding, but as a result of the Spanish-American War in 1898, it had ventured beyond its land borders. It claimed Guam, Puerto Rico and the Philippines as U.S. territories, made Cuba a U.S. protectorate and annexed Hawaii.

America was now an overseas empire, and Roosevelt thought it important for the U.S. to wield the kind of power in world affairs that European empires did. He believed U.S. interests were global interests, and that it was actually good for “civilized” nations—among which he counted the United States—to intervene in other countries’ affairs.

“He believed it was the burden of ‘civilized’ nations to uplift ‘uncivilized’ nations,” says Michael Patrick Cullinane, a history professor at the University of Roehampton in London and author of Theodore Roosevelt's Ghost: The History and Memory of an American Icon.

In addition, Roosevelt was concerned that if the United States didn’t “take a bigger role in world affairs, it would actually decline from being a world power,” Cullinane says. To prevent this, the 26th president made sure the U.S. played a larger role in international diplomacy while at the same time reminding other countries that it was building a big navy—a foreign policy he described as “speak softly and carry a big stick.”

Roosevelt was the driving force behind building the Panama Canal, a waterway that allows ships to travel between the Atlantic and Pacific oceans without going all the way around the tip of South America. U.S. construction on the canal officially started in 1904 and continued until 1914, five years after Roosevelt left office.

When Roosevelt first started trying to obtain the land for the canal, Panama was part of Colombia. America tried to negotiate with Colombia to lease the Panamanian land, but Colombia’s congress rejected the terms.

Roosevelt responded by siding with Panama in its struggle for independence from Colombia. Shortly after Panama formally broke from Colombia in 1903, the U.S. signed a lease for the land to build the canal. Notably, Roosevelt’s 1906 photo shoot at the construction site made him the first president to leave the United States during his term.

Siding with Panama in its war with Colombia was a controversial move because the United States was interfering in the affairs of a sovereign Latin American nation. Soon after leasing the land in Panama, Roosevelt articulated a foreign policy in which he saw the U.S. taking an even more active role in Latin American affairs.

In 1904 and 1905, Roosevelt put forth what’s known as the Roosevelt Corollary to the Monroe Doctrine. President James Monroe’s 1823 doctrine stated that European countries should stay out of affairs in the Americas, where they (and the United States) had long colonized Indigenous nations. According to the Monroe Doctrine, the western hemisphere was part of the United States’ sphere of interests, not Europe’s.

The Roosevelt Corollary took the Monroe doctrine a step forward. In it, Roosevelt said the United States had a responsibility to protect countries in the Americas from recolonization by European powers, and that the U.S. would intervene militarily if it felt it was necessary to do so. This was motivated in part by Venezuela's defaulting on debts that European powers claimed the country owed them, and the concern that these powers could use the debts as an excuse to recolonize land.

At the same time, Roosevelt was reinforcing colonial spheres of interest. He believed Britain’s sphere of interest was East Africa and India, France’s was West Africa and Japan’s was the Pacific (although with Guam and the Philippines, America had begun to enter that region). The Roosevelt Corollary further staked America’s claim on the western hemisphere.

Roosevelt’s precedent of intervening in Latin America influenced the presidents who came after him, and “led to a very unfortunate kind of economic colonialism in Latin America,” says Kathleen Dalton, author of Theodore Roosevelt: A Strenuous Life. Although Roosevelt didn’t use the corollary to take military action in Latin America, many presidents in the 20th century did.

In addition to being the first sitting U.S. president to leave the country, Roosevelt was also the first U.S. president to win a Nobel Peace Prize. He received the award for negotiating the treaty that ended the Russo-Japanese War (1904–1905).

Roosevelt’s role in the negotiation represented a major shift in the U.S. presidency. Before, most presidents had seen American interests as “continental or hemispheric,” Cullinane says. “Roosevelt believed America’s interests were global, and that something that happened as far away as Japan…would have a real impact on American interests.”

It also signaled a new role for the U.S. in international diplomacy.

“The United States by 1900 was the largest world manufacturing power,” Dalton says. “So for the United States to be left out of power conversations in Europe is sort of embarrassing to the United States.” That changed with Roosevelt, whose diplomacy with European countries “helped make the United States a respected world power.”

After Roosevelt helped negotiate the end of the Russo-Japanese War, he also intervened to prevent war between Germany and France over their colonial claims in Morocco.

Before the last quarter of the 19th century, there were no federal laws broadly stipulating who could enter the United States and how, and therefore no such thing as “legal” or “illegal” immigration. That changed with the 1875 Page Act and the 1882 Chinese Exclusion Act, which together restricted almost all immigration from China.

During Roosevelt’s presidency, he signed laws that further restricted who could immigrate to the United States. The Immigration Acts of 1903 and 1907 banned (in the language of the time) the immigration of anarchists, idiots, imbeciles, feebleminded persons, epileptics and paupers, among other categories. In addition, the Expatriation Act of 1907 stated that American women who married non-American men would lose their citizenship.

However, the immigration policy Roosevelt became most famous for wasn’t actually a law; it was an informal diplomatic agreement aimed at easing international tensions. At the time, Japanese officials were upset at the discrimination and violence Japanese immigrants to America faced. In 1907, Roosevelt brokered a deal with Japan in which San Francisco would rescind its new policy of segregating Japanese students from white students. In return, Japan would prevent most Japanese laborers from migrating to the United States.

The Gentlemen’s Agreement, as it became known, helped preserve America’s diplomatic relationship with Japan while continuing to restrict the immigration of people from East Asia to the U.S. In the following decades, these restrictions became even more severe. The 1917 and 1924 immigration acts banned almost all immigration from Asia, severely limiting any migration from the continent until 1965.