Why was there a need to move from the rule of dynasties to nationalism?

Explanation:

Nationalism holds that each nation should govern itself, free from outside interference (self-determination), that a nation is a natural and ideal basis for a polity and that the nation is the only rightful source of political power (popular sovereignty).

although world war i began in 1914, the united states did not join the war until 1917. the additional firepower, resources, and soldiers of the u.s. to tip the balance of the war in favor of the allies. remaining neutral. when war broke out in 1914, the united states had a policy of neutrality.

the answer to your question is:

strong-mayor system