Why were alliances a main cause of world war i?

alliances strengthened demands for self-determination.
nations were less likely to use diplomacy in settling disputes.
if one nation was attacked, the others would come to its aid.

The right answer is:

If one nation was attacked, the others would come to its aid.

The first two options are generical issues that cannot be proven historically.

On the other hand, the obligation to come in aid of an allied nation was a real factor in the origin of the war, that lasted from 28 of July, 1914, to 11 of November, 1918. The historical reason was the assassination of Archduke Franz Ferdinand, heir of the Austro-hungarian empire, by serbian nationalists.

Because of this incident, Austria-Hungary declared war on Serbia, and shelled its capital, Belgrade.

And after that network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe,  dividing it in two main coalitions: the Triple Entente—consisting of France, Russia and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy.

the california missions began in the late 18th century as an effort to convert native americans to catholicism and expand european territory. there were 21 missions in all, lasting from 1769 until about 1833.   the missions were built by the spanish starting in 1769 in order to “colonize” the territory of alta california. so the missions were important because they were the means of settling california. it also wanted to keep other european countries out of alta california. building settlements along the coast would them meet these goals. missionaries wanted to convert the indians to the roman catholic faith. california indians did much of the work to build missions.

i dont know. sorry : (