In the past wars were caused from either religious disputes, an attempt to gain economic or political control over other nations using military force, or a failure to recognize the humanity of a group of people. In the twenty-first century most of these reasons should no longer apply. Most 1st world nations believe in separation of church and state therefor religious leaders have minimal political power over government decisions (in theory). However, there are groups of people who still tie religion to government, and this is where a problem arises; how can you peacefully coexist with someone who has a completely different set of ideas? (The only reason I'm not going to answer this question is because even if I had a really good solution it would be way to difficult to explain here)

As for forcefully trying to control other nations, there is no reason in the 21st century where international diplomacy can be used. Negotiating through organizations such at the UN is the best way to peacefully resolve problems. In this way, violence or threats of violence are unnecessary.

Finally we have one of the main reasons for war, failure to recognize the humanity of others. Most people, normal people, find it repulsive to kill another human being. Therefor, in order the majority of a nation to agree to go to war, the government must demonize whoever they want to go to war against. This is what caused world war two. Government propaganda led to nationalistic pride and eventually a supremacist attitude by Germans and Italians. As long as people keep in mind that we are humans first this need not happen.