Top Five's -- Causes or Events that Led the US to Enter into War.
1. Impressment/Boston Tea Party/Boston Massacre You can lump these causes into one big category, because they are all connected in some way to the people finally saying, "I have had enough". Forced into war or labor, taxed on every product and war fought, and told when you can and can't assemble together will eventually lead to revolution. History has all kinds of examples of the people standing up and taking control from the oppressor. (See French and Russian Revolution)
2. Attack on Pearl Harbor For a World War that started in 1939, the United States stood back and made all kinds of money selling and loaning arms and supplies to all the European countries at war. Attacks on American trading vessels by German U-boats was not enough to get the US to enter into war. However, on December 7th 1941, when the Japanese attacked the US navy base Pearl Harbor in a supposedly surprise attack and over 1,000 American soldiers were killed. The United States declared war on Japan and later Germany and Italy.
3. Manifest Destiny What has a better sound to it? We are taking this land from you and sending you where ever you are out of the way, because I said so or because this is what God wants. Well I guess it would make the conquerors feel a lot better if its all for the sake of God. Which is why most of the cities in California are Spanish and Native American names are spread throughout our rivers and lakes like ghosts from the past.
4. Sinking (explosion) of the Maine A battleship that sank in very mysterious ways was the incendiary to the powder keg that was the Spanish-American War, in 1898. Over 200 soldiers lost their lives in this explosion that was never really investigated thoroughly, but was declared a naval mine and that it was definitely Spain, because it was off the coast of Havana, Cuba. This led to the famous rally cries of, "Remember the Maine! To Hell with Spain!" Many researchers however believe that the Maine sank because of an internal explosion in the hull.
5. Gulf of Tonkin Incident Even though the US had been involved in Vietnam for several years prior to the Gulf of Tonkin Incident in 1964, this event led to a great surge in troops, supplies and money into a war that was just beginning to escalate. The claim of the US military was that the destroyer USS Maddox was attacked by three North Vietnamese torpedo boats and that these attacks were unprovoked because it was international waters. This event gave the President and future Presidents the power to declare war without actually declaring it.
Others that deserve mention: South Carolina Secession and 911/Weapons of Mass Destruction