Franco-American Naval War (Quasi War): John Adams, Federalist (Ancestor Party of the Republicans); See XYZ affair for reason of war
First Barbary War: Thomas Jefferson, Democratic-Republican (Ancestor Party of the Democrats); Tripolitians ransomed American Sailors and tried to blackmail US. When the US refused, the Pasha (equivalent to king) declared war.
Second Barbary War and War of 1812: James Madison, Democratic-Republican; Second Barbary War began the same way as the first only with the Bey of Algiers declaring war. While the War of 1812 can be argued as being a war about sailor's rights, the real motive was Canada.
Mexican-American War: James K. Polk, Democratic; The Republic of Texas had a claim to Land between the Rio Grand and San Antionio(which the US inherited), which was not valid. Polk sent troops to the territory to protect it and the Mexican government sent troops to drive it out.
American Civil War: Abraham Lincoln, Republican; Hostilities began on April 12, 1861, when Confederate forces fired upon Fort Sumter, a key fort held by Union troops in South Carolina. Lincoln called for each state to provide troops to retake the fort; consequently, four more slave states joined the Confederacy, bringing their total to eleven. The Union soon controlled the border states and established a naval blockade that crippled the southern economy.
Korea (1876); Ulysses S. Grant, Republican; Called Choson in 1876, the country attacked and destroyed an American Navy Vessel. The war was fought with similar motives as Perry's visit with Japan.
Spanish-American War and Phillipine Insurrection: William McKinley, Republican; American Naval Ship Maine sent to monitor alleged mistreatment of the civilians of Cuba and protect American economic interests. The ship was destroyed and the press (the real instigators of the war) of the time placed the now doubtful blame on the Spanish. US fought the war to free Cuba. McKinley announced that giving the Filipinos Independence outright would be like simply handing them over the Germans or Japanese (because of the geographic position) and harm American economic Interests.
Haiti; Vera Cruz occupation, Pancho Villa, World War I: Woodrow Wilson, Democratic; Troops were sent in to stabilize Haiti, Vera Cruz was occupied to prevent ships from Germany delivering guns to the government. Pancho Villa made several attacks on Americans (because he was not recognized as President of Mexico) forcing Wilson to send troops to capture Villa. WWI involvement was the result of Germany violating Neutrality rights.
Nicaragua: Calvin Coolidge, Republican; Like Haiti the objective was stabilization.
World War II: Franklin D. Roosevelt, Democratic; Asked for declaration of war after Japan attacked Pearl Harbor. Germany and Italy declared war two days later.
Korean war (1950-54): Harry S. Truman, Democrat; North Korea, in violation of a UN treaty, invaded South Korea. Troops were sent to contain communism.
Vietnam War: Lyndon B. Johnson, Democratic; involvement started after two American gunboats were attacked in the Gulf of Tonkin (the reports have now been proven falsified.).
Grenada: Ronald Reagan, Republican; When a Communist government took over the country, it persecuted the American college students studying there.
Panama, Persian Gulf War Operation Restoring Freedom: George H. W. Bush, Republican; Noriega, leader of Panama was charged with drug trafficking and through Noriega's power, Panama would declare war (Retaining the Panama Canal is allegedly the real motivation). Persian Gulf War was fought to Liberate Kuwait from Iraq and protect Saudi Arabia from an Invasion (also protecting oil interests). Troops were sent to Somalia to assist in the feeding the hungry, after guerrillas shot up UN aid convoys.
Bosnia, Kosovo and Iraq: Bill Clinton, Democratic Party; troops were sent to Bosnia to enforce peacekeeping. Yugoslavia was attacked over genocide upon the ethnic Albanians in Kosovo. Air assaults on Iraq were ordered after failing to comply with UN weapons inspectors.
Afghanistan and Iraq: George W. Bush, Republican; Invasion of Afghanistan was the result of the Taliban ruled government's refusal to hand over Osama bin Laden after the attack of September 11, 2001.