Which of these factors cause world war 1?
a. the sudden economic depression that left europe vulnerable to attack
b. border disputes between fascist and non-fascist countries
c. rising tensions between catholic and protestant nations in europe
d. an alliance system that required allies to aid one another in armed conflicts

D. An alliance system that required allies to aid one another in armed conflicts

Explanation:

The alliance between the UK, France and Russia became known as The Triple Entente, a military alliance that opposed the Union between Germany Austro-Hungary and Italy.

Some years before, a key alliance between France and Russia called the Dual Alliance opposed an increasingly powerful German - Austrian influence in Central and East Europe, as well as in the Balkans.

The alliance system can be compared to a entangled chess game where , two groups stand against, Central powers, against Allied or Entente. In this complex system any move would provoque a reaction of the neighbouring states, so a cascade followed as a reaction to every minuscle event.

The alliance system was one of the main causes since reactions to local events had a multinational event and made the parties act in response.

north east because it has more forest