1. Hitler didn't come to power because of WWI, he came to power because he promised German socialism at a time when socialism was sweeping across Europe.
2. The US was actually winning the war in Vietnam when the Tet offensive occurred. It was desperate offensive by the Viet Cong. It failed in all of it's objectives, they suffered heavy casualties, and actually left the Viet Cong in a very bad position. But the optics of the Viet Cong offensive triggered a wave of negative sentiment at home, and ultimately led for the push to withdraw the US forces from Vietnam.
3. Iraq in 2008 and 2009 had less violent deaths than the US. But Obama's quick pull-out of the troops allowed the insurgents to reestablish themselves in Northern Iraq.
It's easy to say "the world needs to just agree to stop fighting", but when one side in a disagreement is ready and willing to fight and kill, what other choice is there?