The following Presidents were Democrats except as noted.
Democrat James K. Polk was president in 1848 and he could reasonably be said to have started the Mexican War.
Democrat Andrew Jackson--Second Seminole War & Black Hawk War
Democrat Franklin Pierce--Third Seminole War
Democrat Grover Cleveland--Apache Wars
Democrat Woodrow Wilson - President when the US entered WWI
Democrat Franklin D. Roosevelt - President when the US entered WWII
Democrat Harry S. Truman - President when the US entered the Korean War.
Vietnam:
Democrat Harry S. Truman actively supported the French in their Indochina War. That was the real start of the Vietnam war.
Dwight Eisenhower (Republican) was President when the US agreed to supply up to 10,000 advisors to support the new South Vietnam state following the Paris Peace Talks in 1954.
Democrat John F. Kennedy was President when the US expanded it's role in Vietnam to exceed the 10,000 advisory limit, started flying South Vietnamese troops to and from battle in helicopters, and started flying limited air strikes in South Vietnam.
Democrat Lyndon Johnson was President when the US involvement escalated to the point where we had a half a million troops in Vietnam and was conducting and ongoing air war against North Vietnam.
Democrat Bill Clinton--Second Persian Gulf War & US involvement in the Bosnia crisis. Haiti, Somalia, Serbia, Bosnia
Democrat Barack Obama - helped overthrow 3 middle eastern presidents, as follows: The president of Egypt which is an American ally, and Tunisia and Libya in the Arab Spring revolutions.
Although the Iraq-Afghanistan wars started during Republican President Bush, America was first attacked on our homeland. Plus, these wars are nothing in scale compared to WW1 and WW2, when Democrats ruled. I am not saying that we should not have gotten involved in those wars. I'm just saying that history shows that world wars, and the most deadly wars always occurs when Democrat presidents rule.