Most (but not all) Christian churches recognise that there is a time when war can be justified. There are strict criteria that need to be met. We have to use our judgement in any given conflict as to how far they may be met.Before going to war, the following criteria must be met:

• Just Cause: The war must confront a grave evil and by in self-defence, to defend another nation, or to protect innocent life.
• Legitimate Authority: The war must be declared by a lawful, proper authority,.
• Right Intention: The goal must be to promote good or correct a wrong, not for revenge, vengeance, or territorial expansion.
• Last Resort: All peaceful alternatives must have been exhausted.
• Reasonable Chance of Success: The war should not be fought for a hopeless cause, which would waste lives unnecessarily.
• Proportionality: The good to be achieved must outweigh the total evil that the war will cause.

Once a war has begun, there are rules which dictate how it is fought:

• Discrimination/Distinction: Non-combatants (mainly civilians but also those in the military who do not carry arms) should not be targeted.
• Proportional Force: Only the minimum necessary force should be used to achieve military objectives. Methods that are inherently immoral, such as genocide, torture, or using weapons that cause uncontrollable destruction are forbidden.
• Treatment of Prisoners: Prisoners of war must be treated humanely.

Recently thought has been given to what represents a just peace at the end of the war.

• Just Termination: Aims should be limited to the original just cause.
• Reconstruction: Efforts should be made to restore peace, repair damage, and uphold human rights.
• (I have adapted this from an AI generated response to a Google query).