The Spanish-American War was fought simultaneously against Spanish possessions in the Caribbean and the Pacific.  Claiming to defend Filipino rebels, the United States invaded the Philippines and fought the Spanish there, soundly defeating them.  From 1899-1914, the US fought the very Filipino insurgents they were claiming to defend in their invasion in 1898.

First stated by President James Monroe, the Monroe Doctrine became a long-standing tenet of American foreign policy.

The Adams-Onis Treaty, signed in 1819, ceded control of Florida from Spain to the United States. In addition, it settled the boundary dispute between the furthest western settlements of the United States, in what is now Texas, with the viceroyalty of New Spain, (now Mexico). The Treaty is generally considered to be the close of the first wave of American expansion; it was not universally respected, however, and many western Americans refused to recognize its legitimacy—continuing to settle in the territory west of the boundaries drawn up: in modern-day New Mexico and Colorado. 

In 1819, the United States bought Florida from Spain in the Adams-Onis Treaty.

The Rush-Bagot Treaty was a demilitarization treaty signed between Great Britain and the United States following the end of the War of 1812. It forbade the maintenance of more than a few naval ships on the Great Lakes territory between the United States and British North America. As importantly, it laid the foundation for a future agreement to be reached with Canada that has ensured that the northern border of the United States has remained demilitarized for two centuries.

Robert E, Lee was uneasy about secession throughout the crisis that developed following Abraham Lincoln's election, and was not sure about joining the Confederacy.  Lee was offered command of the United States Army before his home state of Virginia seceded, which caused him to join the Confederacy.  All of the other answer choices were Union generals who would come to prominence later in the war.

William H. Seward's detractors believed that the Alaska Purchase was a great mistake, calling it Seward's Folly. 

In 1846, the Oregon Treaty fixed the U.S. border with Canada at the 49th parallel.

The Battle of Little Bighorn was the most decisive victory by Native American tribes against the United States Army. General George Armstrong Custer led his 7th Cavalry across the northern Great Plains throughout early 1876, trying to corral various tribes onto reservations.  On June 25, Custer came upon a large united force of Lakota, Cheyenne, and Arapaho warriors, and the 7th Cavalry was decimated. Custer and all of his officers were killed.  In the aftermath of Little Bighorn, the US Army greatly expanded its efforts against Native American tribes, and began a more concentrated effort to relocate tribes to reservations.

The Mexican-American War was controversially started after the United States annexed the Republic of Texas in 1845. Despite large swaths of opposition, the U.S. Army quickly dominated the Mexican Army. The U.S. conquered present day New Mexico, Arizona, and California, and then proceeded all the way to Mexico City. The Army's commanders, Zachary Taylor and Winfield Scott, quickly became national heroes, and were given the nicknames of "Old Rough and Ready" and "Old Fuss and Feathers." Taylor would be elected President in 1848, and died in office in 1850. Scott was commander of the Armed Forces until the Civil War.