With the Spanish-American War ended, the United States manifested a striking disregard for British naval power (still the greatest in the world) in the strategically critical zone of the Caribbean and Central America. The only two feasible trans-isthmian routes went through Panama, which belonged to Colombia, and Nicaragua. The Frelinghuysen-Zavala Treaty of 1884 (not ratified), which gave the United States exclusive rights to build a canal across Nicaragua, was complemented and made official by the Bryan-Chamorro Treaty of 1914. Panama was the scene of considerable upheaval. In the wake of the scandal-ridden bankruptcy of the Compagnie Française du Canal de Panama (1893), its liquidator, Philippe Bunau-Varilla, made a concerted effort to persuade the United States to purchase the concession that the French had obtained from Colombia.
President Theodore Roosevelt, who was very interested in the project, sought to revive an old idea by forcing the British (at a disadvantage, because of their involvement in the Boer War) to relinquish their share of the control of the canal through the Hay-Pauncefote Treaty. As early as 1880, James G. Blaine had called for replacement of joint Anglo-American control of the canal, provided for in the Clayton-Bulwer Treaty of 1850, by exclusive American control. Blaine was supported by Congress, but the British had refused to agree. The first Hay-Pauncefote Treaty (5 February 1900), approved by the Senate, gave the United States exclusive rights but forbade it to fortify the canal zone. A second Hay-Pauncefote Treaty (signed on 18 November 1901 and approved by the Senate on 16 December) eliminated this restriction.
The United States thereupon entered into negotiations with Colombia to obtain the concession for the canal. This was the object of the Hay-Herran Treaty of 22 January 1903, which was approved by the Senate but rejected by Colombia. Rather than seize the zone by force, Roosevelt preferred to encourage Panama's secession from Colombia. Coordinated by Bunau-Varilla, this action was taken on 3 November 1903; and by 6 November the United States, which had sent warships to the area, recognized the new republic. Bunau-Varilla was sent as Panamanian minister to Washington; and on 18 November he signed the Hay–Bunau-Varilla Treaty, whereby the United States was accorded the concession in perpetuity of a zone ten miles wide, with complete sovereignty and fortification rights. The Senate accepted the treaty on 23 February 1904.
In order better to protect the Canal Zone (the canal itself was completed in 1914), the United States set out to maintain order in the Caribbean. With Cuba there was initially only a unilateral decision made in 1901 (in the form of an amendment by Senator Orville H. Platt to the Army Appropriations Bill) according itself the right to intervene there. The Cubans were obliged to incorporate the Platt Amendment into their constitution. A treaty of 22 May 1903 later confirmed the amendment. This interventionist policy in Latin America, which President Woodrow Wilson greatly extended, gave rise to unilateral actions by the United States (based on the Roosevelt Corollary to the Monroe Doctrine, on 6 December 1904). Referred to as the "big stick" policy, it ultimately resulted in the landing of U.S. marines in Nicaragua on 14 August 1912. However, the Senate refused to ratify a treaty giving the United States a naval base in Nicaragua.