And you don't need all of your cities to be at 100% production to be able to engage in warfare.
This really can't be said often enough. 100% is a reward for hard work, it should not be considered a natural state for the city. You carefully feed it and tax it at just the right rate, you do courtier work and over the course of a year or two it grows into this giant money tree. But in the mean time, your enemies will want to chop it down.
Just look at how cities in the middle ages were
completely razed. Baghdad comes to mind. Population of 2 million in the 13th century, razed to the ground and turned into a barren wasteland for hundreds of years. The population didn't even reach six digits again until the 20th century.
