The major condition that led to the Thirty Years' War was the Protestant Reformation and the religious divisions among the various German political entites.
The Peace of Augsburg in 1555 had allowed each of these political entities to choose its own religion. This had led to a situation in which there were hundreds of small states, each with its own religion. Many of these states were interested in expanding.
By the early 1600s, religious tensions had resurfaced. Various religious sects were trying to expand and that was causing a great deal of tension. It was from these conditions that the Thirty Years' War, which began as a Protestant-Catholic religious war, arose.