If someone builds a house, he must spend a lot of money. However, afterwards there is a house, a valuable thing standing against this money.

If someone wags a war, he must spend a lot of money. But afterwards, he has trouble. No matter who was victorious, after a war you have trouble.
So nothing valuable stands against the money spent for the war.
Moreover, a lot of valuables, for which money was spent, were destroyed during the war.

Nevertheless, all this money is valid currency. So there is more money on hand, but less to buy for it, and consequently money is devaluated.