Taxes
Each nation you hold has its own tax. More tax means more money, but grumpier people.
Taxes are your main income. Every nation you hold has its own tax slider in the War Budget panel.
People accept a fair tax without fuss. Raise it too far and they start grumbling and dodging it. Push much further and some pack their bags and leave the country for good.
Small nations put up with higher taxes than big ones. The slider's colours show how people feel about each rate. You can see what a new rate would bring before you set it.