Why did fredrick william of prussia build a large standing army?

Frederick William was to spend the rest of his life building the Prussian army into Europe’s best fighting instrument. Realizing that Prussia’s military and financial weakness made it dependent on the relations between the great powers, Frederick William resolved to make his state financially independent.

In 1713 Prussia’s armed forces numbered 38,000 soldiers, supported in large part by foreign subsidies. When Frederick William died in 1740, he left his son an army of about 83,000 out of a population of 2,200,000, a war chest of more than 8,000,000 taler, and a Prussia that had become the third military power on the European continent, after Russia and France.