How did the alliance system cause world war 1?
If country A attacks country B, then A's allies also attack country B, and B's allies all come to the defense of B.
In the years preceding World War I, the alliance system and the arms race worked together to bring Europe to the brink of war -- and then into war itself. The alliance system, designed to prevent war, actually forced nations into a war that did not initially involve their economic or political well-being. The arms race among European nations was also a factor inasmuch as it gave confidence to each side that, in the event…
The United States was bombed by the Japanese at Pearl Harbor which cause a declaration of war but Germany had an alliance with Japan and it had forced the United States to declare war on Germany too. Then Germany had an alliance with Italy which cause the United States to declare war on yet another country. Then the world was at war and therefore World War 2 started.