I think this is a homework question but to answer it a bit:

Romanticism was the movement based on subjectivity, emotion, it was a 'natural' movement looking at artistic and, again, emotional and sentimental perspectives.

This movement was expressed in different ways but one of the most famous ones is with Nationalism. The glorification of nationhood, nations, and these kind of emotions. In the 19th Century we see the rise of European nations, wars, conflicts between peoples (as a consequence of the French Revolutions and the Napoleon Wars). The 19th Century is the start of European nationhood ofc.

The 20th century was the extreme version of this 19th century, it was not only about making your own nation, forming an own state. The 20th century was the era where peoples thought their nation, and their people, is the best one in the world and other ones should be conquered or even worse.

It is ofc a bit wrong to say this all is the consequence of Romanticism. But this movement is an expression of the sentiments in this era among others.