In March of 1938, Nazi Germany led by Adolph Hitler gave an ultimatum to Chancellor Schuschnigg of Austria to resign and allow a new Chancellor of Germany's choosing to take over or Hitler's troops would march into Austria.
With his back against a wall, Schuschnigg did resign and Dr. Arthur Seyss-Inquart, a Nazi puppet, took control. Immediately, he ordered the Austrian army to offer no resistance, and then invited German troops to enter Austria. Hitler achieved his Anschluss - the union of Austria and Germany.
When Austria became a part of Nazi Germany, little Czechoslovakia, a democratic country formed out of the victory of the Western Allies over Germany in the first World War, found itself surrounded on three sides. And though Britain and France had barely reacted to the previous Anschluss, Hitler knew full well he could not invade Czechoslovakia without a reason.