What factors would trigger war among major european powers

I'm assuming your question is in reference to matters before World War I.

I'll answer based on choices I've seen elsewhere:

changing relations between powerful nations  one nation’s invasion of another nation resentment from past conflicts

Explanation:

The changing relations between nations were especially seen in the system of alliances that developed.  The alliances and military plans which were put in place before World War I presupposed a major war between the countries which were tied together with alliances.  The Triple Entente lined up  Britain, France and Russia as allies.  Germany, Austria-Hungary, and Italy aligned with one another as the Triple Alliance.  Germany thought if a war began it would need to fight on two fronts -- west and east.  So German Field Marshall Alfred von Schlieffen drew up war plans that said attack France first, quickly, and then hold that territory while deploying forces to contend with Russia in the east.  When World War I began, that's the plan German forces put into effect.When World War I began, it was triggered by Austria-Hungary threatening war against smaller independent nation Serbia, after the Austrian archduke and his wife had been assassinated in Serbia.  The Russian army mobilized to protect Serbia (an ally of theirs).  The Germans then declared war against Russia for the troop mobilization and looming invasion of Triple Alliance territory that they saw taking shape.Germany and France had been at odds for a long time, and the establishment of the German Empire in 1871 had come after the Germans defeated France in the Franco-Prussian War.  So yes, there were past resentments in place.

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Every statement this is because of the same law