Which of these factors helped cause world war 1 A. Rising tensions between Catholic and Protestant nations in Europe
B. The sudden economic depression the left Europe vulnerable to attack
C. Border disputes between fascist and non-fascist countries
D. An alliance system that required allies to aid one another in armed conflicts

The factor that helped cause World War one was an alliance system that required allies to aid one another in armed conflicts.

Explanation:

The conflict was a result of tensions between the European powers along with the crisis of the balance-of-power system that splited Europe into two sides. One the one hand, one side was made up of Great Britain, France and Russia, which was called the Triple Entente. It strove seeking to preserve the weak balance between the European great powers. On the other hand, the German Empire, Austria-Hungary and Italy (Central Powers) formed the other camp.

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confederacy: supported slavery, believed states should be able to nullify federal laws, supported low tariffs

union: everything else