Dukie,
Jesus preached against war but he also said in the gospels “Render unto Caesar the things which are Caesar’s, and unto God the things that are God’s.”
Jesus said it is inevitable that wars will continue until He returns (Mark 13:7-8), and He did not oppose earthly governments or their right to maintain armies (Matthew 8:5-10). Other New Testament passages accept the necessity of maintaining armies and the worthiness of military occupations (Luke 3:14, Acts 10:1-6)

Just War Theory
Clearly, the Christian ideal is total elimination of war and brotherly love among all people. However, in this imperfect world, war may be forced on those who do not desire it. St. Augustine (354 - 430) and St. Thomas Aquinas (1225-1274) are primarily responsible for formulating the theory of the Just War which has remained the majority Christian approach to war to this day. There are many variations on the just war theory, but these are the basics:

There must be a just cause for the war.
War must be waged only in response to certain, grave and lasting damage inflicted by an aggressor.
The motive for war must be advancement of good or avoidance of evil.
The ultimate objective of war must be to bring peace.
Revenge, revolt, a desire to harm, dominate, or exploit and similar things are not justification for war.
Every possible means of peacefully settling the conflict must be exhausted first.
There must be serious prospects of success; bloodshed without hope of victory cannot be justified.
The war must be declared by a legitimate authority. Private individuals or groups should seek redress of their rights through their governments, not by acts of war.
The war must not cause greater evil than the evil to be eliminated.
Non-combatants (civilians) must not be intentionally harmed.
Prisoners and conquered peoples must be treated justly.