War is only allowed as a last resort. “ Whenever there is lawlessness and decline of morality the miscreants should be put down to protect the innocent” is how it is put in the classics. This includes self preservation, to protect yourself from invaders. Then all bets are off. Even cornered rats fight.
Reasons would be
1. Patriotism
2. A belief that there is no other option available
3. Because the cause for which you are fighting is just and valid
4. To defend your beliefs, values, country and family which is best done
a) by killing some other mothers son/daughter
b) on somebody elses territory
1. Patriotism is a false sense of being attached to that small part of the world where you were born and trying to somehow defend it from other people who want to live there.
2. There are always other options. You're never forced to doing 1 thing there are always choices, even if you think that you are cornered.
3. War is never just. Attacking other people is never just.
4. Why do you need to physically defend your values? You have the right to defend yourself if someone uses force against you, but war is always started by someone and thus it's never justified. If someone has other beliefs than you, you have no right to attack and kill him, thus war should never be supported.