We have a good advantage over the other civilizations if we are a Republic, we'll have lower war weariness. So we can fight a war and they'll get hit harder, plus are war weariness will be counteracted by our luxuries and improvements and wonders. We'll only be losing a bit of time for things like roads, which will be very little. There fore I believe that we should be in a Republic for as long a time as possible, and if we get Universal Suffrage, I'd support Democracy.
