This is an offshoot of the Conquered! campaign seed. It gives you a 'hook' to introduce the characters to the campaign.
A good hook is to start the PCs out in their home village/town when the news comes through of the defeat of their armies and the rapid advance of the enemy. This gives the players the chance to decide whether to stand and fight (bad idea – if the regular army can’t stand against them, what chance does a small street rabble have?), or whether to hide any weapons they may have (wealth too – armies usually loot during the conquest phase). Make sure they specify where they are hiding the weapons.
When the invaders arrive, they gather all the townsfolk together and announce that they will be confiscating all weapons (and armour, if appropriate). Anyone who has weapons is to make it known, and they will then be escorted to fetch them. After they have the chance to surrender their weapons, the townsfolk are still held in the market square while the soldiers search a sample of the houses. If any house is found to have a concealed weapon, it is looted and burned down by way of punishment. (The invaders are emphasising their ruthlessness, without killing anyone so that they don’t immediately start blood-feuds.) While the first house is burning, the townsfolk are offered a chance to “remember” where there may be weapons. (The troops will probably search 1 house in 6: roll to see whether any house the PCs specify they hid a weapon in is searched, then modify the search roll by how well they hid it.) A clever PC, of course, will have specified that their weapons are hidden in some place not identifiable with them – buried it on common ground, for example, or hidden in a pig-sty where they have plausible deniability even if it’s found. If the PCs lead a massed rising when the townsfolk are assembled in the square, they should be able to overcome the comparatively small force which was sent out to confiscate weaponry – though the soldiers will fight to the death and people they have known all their lives will die before the fight is over. A large force can usually overwhelm one with superior skill and weaponry but significantly smaller numbers. (That should be a lesson to them: they will normally find themselves on the other side of such odds.) But then the main enemy will take notice, and turn up in force to apply their own overwhelming strength to destroy the town and take the entire population into slavery, as an object lesson to others.You need to leave the PCs an escape option – before the main force arrives, during the fighting, or after their capture as they are being taken to the labour camp.
Of course, leading an uprising against armed soldiery requires a lot of charisma. Most likely result is that the spotty youth who tries it will need to find an escape route pretty quickly as the townsfolk stand around stolidly, refusing to risk their lives on the say-so of such a nonentity.
If the PCs do somehow trigger a rising and the town is subsequently attacked in overwhelming force, it will change the flavour of the campaign. In WWII terms, they will need to escape to the country and form a maquis rather than the urban (or suburban) covert Resistance force. This of course can happen at any time, but if you’d rather start it as a covert group campaign, it doesn’t matter what the PCs say: they won’t trigger an uprising. You as GM can rule that even a critical leadership roll from a PC doesn’t outweigh the threat of those sub-machine guns for the common townsfolk.