Austria-Hungary and Serbia were foes during the first part of the 20th century.
A leading politician (Archduke Ferdinand) from Austria-Hungary was visiting Serbia in 1914 when a Serbian national assassinated him.
In retaliation, Austria-Hungary declared war on Serbia.
Russia, being in Alliance with Serbia, found themselves at war with Austria-Hungary.
Germany and Italy, in Alliance with Austria Hungary, found themselves at war with Russia.
England and France, in Alliance with Russia, found themselves at war with Germany and Italy.
Within four years, the United States felt attacked by the Germans and declared War on Germany.
The conventional historic explanation is that the Great War was the result of treaties of alliance.
This, however, would be the first major war after the invention of automatic weapons, airplanes, chemical warfare, and electricity; a very deadly concoction.
DECISION: To be in an alliance has a major benefit: your friends are obligated to aid you in war. It also has a major drawback: you are obligated to aid your ally in war. Given the pros and cons, is it wise to make a treaty of alliance?