The United States fought numerous wars between the Civil War and the present. Why did the United States fight so many wars? How did they change American society and politics?
Some wars that could be included in the answer
1. The War of 1898
2. The First World War
3. The Second World War
4. The Vietnam War
The United States has fought many wars for many reasons, but the conflicts of the Cold War era tended to be defined and justified on the basis of the need to contain the Soviet Union and the spread of communist regimes and insurgencies that were seen as threatening to the non-communist world. In examining the span of time, and the myriad conflicts in which the United States became militarily engaged since the Civil War, though, the reasons for that engagement are as numerous as the wars themselves. The War of 1898 -- the Spanish-American War -- was fought over the perceived necessity of eliminating European influences in the Western Hemisphere while elevating the global position of the United States, particularly with respect to the vast Pacific region. The United States only very reluctantly became engaged in World War I, and only then was President Woodrow Wilson able to justify such intervention in a European conflict when Germany's policy of unconstrained submarine warfare in the Atlantic resulted in the May 7, 1915, sinking of the British ocean liner Lusitania causing the deaths of almost 2,000 people, including 129 U.S. citizens. Even this act by Germany, however, was insufficient to rally the American public in support of entering the war. The final straw, so to speak, was the revelation of what became known as "the Zimmerman Telegram." That communication from Germany's foreign minister, Arthur Zimmerman, to that country's ambassador to Mexico suggesting an agreement between those two countries that would cede U.S. territory to Mexico (some would say "cede U.S. territory back to Mexico") in exchange for Mexico's support for Germany in the war. The Zimmerman Telegram, combined with U.S. affinity for the French dating back to that nation's support of the American Revolution, and with the continued threat posed by German submarine operations finally proved sufficient to escalate U.S. public support for the war, enabling Wilson to send American troops to that most futile and costly of endeavors.
American antipathy towards engagement in European conflicts returned to the fore following the experiences of World War I, and Franklin Roosevelt would find himself similarly constrained in his desires to more openly confront the growth of Germany's military threat to Europe. Roosevelt's material support for Britain's lonely fight against Nazi Germany -- the Battle of Britain occurred prior to official U.S. entry into World War II -- would only be permitted to overcome isolationist sentiments and emerge as a full-scale military effort following the December 7, 1941 attack on the U.S. naval installation at Pearl Harbor, Hawaii and the concurrent Japanese invasion of the Philippine Islands (with the attendant capture and imprisonment of thousands of American soldiers). The American entry into World War II was a matter of the survival of the very nations of Europe -- Britain and France -- that compelled American entry into the "war to end all wars" that had preceded it thirty years before, but it was the Japanese attack on Hawaii that provided Roosevelt the casus belli he needed to compel American engagement in that most deadly of conflicts.
Finally, the war in Vietnam was the quintessential Cold War conflict fought in the name of containing communism while preserving the independence of South Vietnam. The United States initially became involved in Southeast Asia as part of a quid pro quo with France at the end of World War II. France had desperately hoped to reconstitute its colonial holdings in Indochina following Japan's defeat, and would only agree to support the establishment of the North Atlantic Treaty Organization -- ironically, an alliance intended to protect it and the rest of Western Europe from a Soviet invasion -- if the United States would support French efforts in Southeast Asia. When the French were decisively defeated by the Viet Minh in 1954, politicians from both political parties in the United States, including a young senator from Massachusetts named John F. Kennedy, argued that the fall of South Vietnam to the communist regime now controlling North Vietnam would result in the spread of Soviet-inspired communist dictatorships throughout Southeast Asia. Vietnam, consequently, became an important and high-profile test case of the American policy of containment.
In short, the United States became involved in overseas conflicts for a variety of reasons and under varying circumstances. The greatest consistency involving such military interventions involved the Cold War imperative of containing and confronting the Soviet Union and its allies.