What topic is often used as a theme for studying world history since world war i?

The topic that is often used as a theme for studying world history since World War I is decolonization and nationalism.

Decolonization and nationalism were used because these are closely related themes to the causes and effects of World War I. Decolonization was an effect of the end of World War I, while nationalism is the factor which helped nations conquer the odds brought by the war.

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explanation:

on this day in 1913, henry ford installs the first moving assembly line for the mass production of an entire automobile. his innovation reduced the time it took to build a car from more than 12 hours to two hours and 30 minutes.