Poland did not have an especially formidable military - far from it. Instead, there were a couple of reasons why Poland was invaded, generally having more to do with politics than with technical military advantage.

First, Poland (and many central European states more broadly) contained substantial German minorities, and fit into the National Socialist ideological program of conquest, which was based around (1) uniting these German-speakers within a single state, (2) enslaving Slavs, and (3) destroying the Soviet Union. In other words, conquest of the East was the primary goal, and war on the Western front - with the important exception of contested areas on the French border - was in support of this. (Indeed, until relatively late, Hitler believed that the British might serve as an ally against Soviet and American power.)

Second, Nazi brinksmanship had been incredibly successful up until that point. The Western powers had assented to numerous annexations, and Hitler believed that - in spite of a recent territorial guarantee by Britain to Poland - that this time would be no different:

His experience of Western appeasement in 1938 convinced him that neither Britain nor France would seriously fight for Poland. This conviction dominated Hitler’s thinking throughout the crisis which led to war. The decision to attack Poland can only be understood in the light of this conviction. The war with the West, if it came to war, would come not in 1939, but in three or four years as planned, ‘when the armaments programme will be completed’. German leaders clung to this timetable uncritically. In May Ribbetrop told the Italian Foreign Minister, Count Ciano, that ‘it is certain that within a few months not one Frenchman nor a single Englishman will go to war for Poland’. In August Hitler told Ciano the same thing: ‘the conflict will be localized … France and England will certainly make extremely theatrical anti–German gestures but will not to go war.’ (Overy, p. 65)

This was the result of a failure of communication. Britain had been trying to signal its willingness to go to war in defense of further violations of the state system, and if this signal had been interpreted correctly, the German leadership may well have made different decisions.

Third, the Soviet Union had grown more open to cooperation, at least in the short term. Under the diplomatic leadership of Maxim Litvinov, the Soviet Union had been eager to cooperate with Western powers to contain German expansion, and extended numerous offers to draw harder lines in the sand, earlier, or even to make plans for a pre-emptive attack on Nazi Germany (though it's hard to say how serious these latter proposals were). However, Western powers were unsympathetic to and suspicious of the Soviet Union, and it wasn't uncommon among conservative politicians to have some sympathy for fascism as an anticommunist force if nothing else. Soviet policy thus moved away from cooperation from the West and towards attempting to find a separate peace with Berlin. The dismissal of the Western-oriented (and Jewish) Litvinov with Molotov signalled a new willingness to negotiate with Nazi Germany, which resulted in the Non-Agression Pact dividing Poland.

For a general narrative history of the origins of the War, I would suggest Richard Overy's The Road to War. For the Russian angle, see Geoffery Roberts, The Soviet Union and the Origins of the Second World War. For some background on the general Nazi ideological vision of conquest, Aristotle Kallis' Fascist Ideology: Territory and Expansionism in Italy and Germany, 1922-1945 is a good place to start.