A war creates more harm than solving the problem.
A war is initiated primarily because one of the parties involved wants to establish superiority over the other. Sometimes, when a country was not ruling its people properly and making them suffer, other countries stage a war against that country and free the country of the bad rule. This happened in the middle east in the last decade.
Some times war occurs on a third country when two other superior countries fight for it. Another situation, when people of a country seek help from a neighbour to free their own country of bad rule.
In any case, during the war people die. A lot of money is spent from all sides. The peace of life is totally disturbed. Government may impose taxes for collecting more funds. A lot of families get dislocated. Property of people gets damaged. No one helps them to restoration. A lot of diseases can spread. Soldiers and their families live under a lot of tension.
Enmity between countries does not really die. People still have those feelings. Like in case of china and India or China and Pakistan. India had suffered very badly after its wars.
After first world war, Germany came back strong. After second world war Japan built themselves to surpass major economically developed countries. Instead of warring, they focussed on building their nation. So prospered.
War kills one side and forces them to accept the winner. The issue between the parties comes up again later. The actual issue remains unsolved.
War promotes weapon industry, terrorists, opportunists and dictators.
Even when America wants to fight, the Americans do not want to participate. As their taxes would increase and their national resources are utilized else where.