Just to add a bit to the very good answer you've already received, there was a strategic level to this. According to Gerhard Weinberg's A World at Arms, Hitler's strategic thinking went something like this:

Main goal: conquer Russia, and enslave and starve to death its population (with the exception of the Jews, who were to be exterminated first) so that the land could be occupied by pure-blooded Germans.

Problem #1: Resistance from Poland and Czechoslovakia, combined with a threat from England and France.

Problem #2: Going to war with everybody at once is what nearly took out Germany in WW1 (Hitler believed that the Western front had collapsed because of a "stab in the back" in the home front, and never accepted the fact that Germany had actually lost the war).

Solution: First, ally with Russia to get the resources needed to knock out Czechoslovakia and Poland, and then do so. Then, while still allied with Stalin for Russia's resources, defeat France and England. Once that was done and Russia was alone on the continent with Nazi Germany, attack and defeat Russia.

And that's pretty much exactly what happened, up to the "defeating Russia" part. Hitler managed to keep himself only fighting on one front at a time, drove France and England off the continent, and then redirected his armies towards Russia. Of course, once they got there, it turned out that the Nazi racial belief in the inferiority of the Slavs and the belief that the Soviet government would just collapse as it did in 1917 were somewhat mistaken, and then the fact that they hadn't actually defeated Britain or its empire bit them in the hindquarters in a REALLY big way.

As my thesis supervisor put it years ago, Germany was stupid enough to declare war against the entire world...twice.

Although Hitler is nowadays mainly known for the Holocaust, his ideology went a bit further than that. To put »national socialism« in a nutshell, the idea is that natural resources are limited and three different types of human races are fighting for the largest possible share. You could spot the differences between the races by their handling of culture: A master race or »Herrenrasse« would be able to found a culture; an inferior race would still be able to keep it up if shown and ordered accordingly. However, the worst races were considered those who could neither found nor keep up a culture; their raison d’être would be destroying the other races’ efforts. Jews were considered the worst proponents of this group as they’d not only destroy foreign cultures but, thanks to their European looks, could do so without people noticing.

Germans, as you probably know or by now would have guessed, belonged to the Germanic master race, encompassing also the British and the Scandinavians; Slavic people – as in Poland or Czechoslovakia – were deemed inferior and suitable only to serve the Germans’ needs. However, since Germans were considered a »Volk ohne Raum« (people without space), they’d need to conquer more »Lebensraum« or space to live in. This ideologeme is also known as »Blut und Boden«, blood and soil. Given that there were Germanic peoples to the North and West, Romance people to the West and South, the natural way for expansion seemed to be the East. These areas, once conquered, were planned to be Germanized, e.g. the sizable Jewish population alongside the Slavic peoples were supposed to pave the way for Germans and then be eradicated.

Hitler tricked Stalin into believing they were allies, so the USSR backed Hitler’s move towards Czechoslovakia, and six weeks after the German invasion of Poland – not all of Poland though, but only to the demarcation line following the Vistula, Narev, San rivers –, Stalin's troops invaded the East of Poland (which they never actually gave back; Poland was instead »compensated« with German land after World War II and had to deal with huge numbers of internally displaced persons). Right from the start though, the Nazis planned to conquer parts of Russia, too, probably as far East as the Ural mountains, seperating Europe from Asia – all in the name of »Lebensraum« for a race that they deemed naturally superior.