The United States wanted to remain neutral in both the World War's when they initially began. It was not until 1917 that the United States joined World War I. This was 3 years after the war initially started. The only reason the United States even joined the war was due to the interception of the Zimmerman Telegram, which revealed a secret alliance that Germany was trying to form with Mexico in order to invade the US.
The US was reluctant to join World War II as well and did not officially join the war until December of 1941, 2 years after World War II began. The US decision to officially join was caused by the Japanese attack of the Pearl Harbor military base in Hawaii.
At the beginning of both World War I and World War II, the initial policy of the United States was to "remain neutral", since the United States during these times was a very "isolationist" nation.
It stemmed from the annexation of the Republic of Texas by the U.S. in 1845 and from a dispute over whether Texas ended at the Nueces River (the Mexican claim) or the Rio Grande (the U.S. claim).
