Put them in jail. If you're in a position to execute someone, then you're in a position to imprison them. These are not supervillains who can break out of Arkham Asylum every time there needs to be a new round of villainy.

Would England have freed the United States without violence?

Why wouldn't it? It freed Canada.

Would Hitler have backed down from controlling Europe if you held a sign for long enough? How about Putin? Or Netanyahu? Or Pol Pot? Saddam Hussein? Hideki Tojo?

At these points you're in a state of active war with another country, execution is not an option. You need to fight the actual war at that point.

Afterwards, though, once you've won the war and have captured the leaders and war criminals and such? No need to execute them, the war has been won. Imprison them.

There are countless examples from history where meeting violent with violence is the only answer. They bring it. You either fight back or you die.

Those aren't the examples at issue, though. Unless you think a guillotine is a battlefield weapon?