Without further context, I would say it could be both. The conqueror may have threatened to nuke them from orbit (external force). Or they could have been caught unprepared and run out of food within two days, due to not building up stocks (their own fault). Or it hadn't been raining for three months in a row, so they run out of water, or there was a flood approaching, or a hurricane had devastated the town (force of nature). Or they just realized that the conqueror was a saint, bringing justice, peace, and candy for everyone, and in light of these facts giving up was the most sensible thing to do.
In any case, forced does imply force, but it does not identify it all by itself.