Why did many european nations quickly join world war i following the assassination of archduke ferdinand in 1914?
they were ready for war.
they had pledged to fight with other countries.
they wanted to prevent more assassinations.
they were ordered to by the united states.

I think that the best answer is the second one:
They had pledged to fight with other countries,

although in a way, the first is also appropriate, as Europe was witnessing a lot of internal tensions at the time. But the main and more direct reason was that they had pledged to fight with others.