Question

Why did Hitler most likely believe Germany had a right to invade other countries?

The Treaty of Versailles did not state that Germany could not invade other countries.
The German economy relied on selling exports, and these nations were no longer purchasing them.
The Third Reich was a struggling government that needed to gain land.
The concept of nationalism meant keeping all Germans together.

Answer 1

The concept of nationalism — in particular Nazi ideas of uniting all German-speaking peoples and creating "Lebensraum" (living space) — gave Hitler an ideological justification for invading other countries. He framed annexations (Anschluss, Sudetenland, etc.) and territorial expansion as restoring and protecting the German nation, not as lawless aggression.