So as far as my underestanding goes, one of the conditions upon which the Western powers agreed to the Munich treaty was that the remaining Czechoslovakian state would be guaranteed sovereignity and integrity of its new borders. Yet in March 39, barely half a year after the agreement, we see Wehrmacht marching into Prague and effectively annexing the entire country. What pretext did Hitler use for this? How did the Allied powers react, and why did they let Hitler get away with yet another act of agression and promise-breaking?
As you might know, The Munich Agreement and the First Vienna Award partitioned Czechoslovakia in 4 different entities, and the Second Republic contained a lot of non-Czechs (Slovaks, Ukrainians etc), who the Republic had to give greater political power to. The Slovaks formed their own autonomous government, as did the Ukrainians. So Czechoslovakia was essentially 3 different states now with a very weak central government.
Also, Czechoslovakia didn't just lose its borders, they lost a major portion of their defensive fortifications (which they paid dearly to build) and their richest areas. It would be effectively unable to mount any meaningful resistance.
On 12 March 1939, the Slovakians declared independence under Tiso, who joined the Axis as a satellite state. 3 days later, Hitler threatened to bomb Prague if the Czech part didn't capitulate, and Hacha (the Czech minister) agreed after lightning fast talks and a heart attack. Meanwhile, the same day, the Ukrainians also declared independence and were invaded and annexed by Hungary. The Allies had neither the will nor saw it worthwhile to defend such a weakened state. Their guarantees didn't cover the Slovaks and the Ukrainians, and Czechoslovakia was essentially reduced to Prague and the surrounding province.