The 30 Years War was at base a war between the Catholic HR Austrian emperor and his Catholic allies and the much less unified Protestant states of the Empire. Left to themselves the war might have been relatively short.
The problem is that outsiders kept jumping in whenever the war looked like somebody might win. The Spanish were involved from the beginning, occupying the Palatinate. They would doubtless have been more involved, but they generally had their hands full losing their 80 Years War against the Dutch. Nevertheless Spanish armies were heavily involved in several major battles of the war in Germany.
The Danes, Swedes and French also got involved at different times. The Brits somehow managed to stay out of it, possibly because they were saving up strength for their own civil war.
Never forget the War had what was called THE DANISH PHASE.