What is one factor that contributed to world war 1?

Traditionally, the murder of Austria-Hungary's crown prince Franz Ferdinand was the thing that started it all. Another important factor was the resulting invasion of Serbia by Austrian troops, which started a whole chain of other countries getting involved because of offensive/defensive mutual support-treaties that bound them. Russia got involved because it felt bound to support Serbia, Gemany was bound to support Austria-Hungary and Russia and France also had a mutual support treaty. Germany invaded France as a preventive strike since (watch closely!) France would have otherwise been bound to attack Germany when Germany attacked Russia in support of Austria which had attacked Serbia which had called in the support of Russia.

It is one of history's little ironies that the Serbian troops needed no-one's help as it turned out. They quickly kicked back the Austrian troops across the border and if everyone had just minded their own business, the Austrians and Serbians probably would have come to a quick face-saving agreement.