Economics is relative. If you go to war and win, you are wealthier than the enemy who lost. If you are confident of victory, the "cost of war" thus matters less, because you get on top anyway.

Of course this only applies in a vaccuum. If there are any nations you don't attack and win, they can just build up their economy while you tank yours fighting and then swoop in to defeat you on the cusp of your victory.

Basic strategy in the game of Civilization. The rest of the world gangs up against a warmonger and unless they can promptly achieve world domination, ultimately self-destruct.