Why did Hitler attack Poland first in World War II?

Although Poland was not a powerful state militarily, it posed a threat to Germany. In addition, the USSR and Germany signed an agreement on the delimitation of spheres of influence. Thus, the Germans knew that Soviet troops would attack Poland. They (the Germans) did it for two reasons:

1) Germany feared that Soviet troops would occupy most of the territory of Poland. That is, they did not want to waste time;

2) A simultaneous attack from both sides will practically destroy the Polish army. If Germany had to fight Poland alone, it would take much more time and resources.