After its victory in the Spanish–American War of 1898 and the subsequent acquisition of Cuba, Puerto Rico, the Philippines, and Guam, the United States had gained a colonial empire. By ejecting Spain from the Americas, the United States shifted its position to an uncontested regional power, and extended its influence into Southeast Asia and Oceania. Although U.S. capital investments within the Philippines and Puerto Rico were relatively small, these colonies were strategic outposts for expanding trade with Latin America and Asia, particularly China. In the Caribbean area, the United States established a sphere of influence in line with the Monroe Doctrine, not explicitly defined as such, but recognized in effect by other governments and accepted by at least some of the republics in that area. The events around the start of the 20th century demonstrated that the United States undertook an obligation, usual in such cases, of imposing a "Pax Americana". As in similar instances elsewhere, this Pax Americana was not quite clearly marked in its geographical limit, nor was it guided by any theoretical consistency, but rather by the merits of the case and the test of immediate expediency in each instance. Thus, whereas the United States enforced a peace in much of the lands southward from the Nation and undertook measures to maintain internal tranquility in such areas, the United States on the other hand withdrew from interposition in Mexico.