Czechoslovakia's defenses were formidable. Strong underground bunkers in a network along the border with Germany.

That border region had a majority of Germans living in it though (Sudeten Germans, and this area is the Sudetenland)

Hitler started by making outrageous claims against the Czechoslovak government, that the Germans in Czechoslovakia were being marginalized, and eventually he even alleged horrible atrocities against Germans. All as a means of propaganda.

Add to that the fresh memory of WWI, and no one (except Hitler) wants a war.

Now Hitler's argument was basically:

• Czechs are committing atrocities against Germans in Czechoslovakia.
• We need to protect those Germans.
• We will avenge the Germans by conquering all of Czechoslovakia, and destroying them. If you want to go to war (England and France) so be it.

The "compromise" was to just give Hitler the Sudetenland.

Beneš (Czech head of government) agreed, because going against the Allies would leave Czechoslovakia to deal with Hitler all alone (also part of Hitler's plan)... so to the Czechs (at least to Beneš, many disagreed) it was clear: lose the Sudetenland, or lose all. And to the Allies it became: Give the Sudentland (a tiny sliver of land) to Hitler, or risk WWII (which no one could imagine even Hitler wanted, since WWI was so bad, and only 20 years previous).

Once that was accomplished Czechoslovakia was defenseless. The Germans marched in unopposed and took the rest.