What event was the spark that ignited world war i?

The treaty of versailles was unfair towards Germany, so that would probably the spark on the western front.

Hitler prepared rather long for an all out war, he took parts of former germany back in peace. Later the USSR (union of Russia and a lot of other countries) took Poland together. (they had a non-attacking treaty). With attacking poland, France and the UK declared war and Germany took over France, Belgium and Holland almost instantly