If a certain country invades its neighbor killing its citizens to gain more territory would you do this fictional act?

If you had said the country’s leader and its military leadership, then I would consider it. But the entire military? No. That would include anyone drafted or otherwise pressed into service regardless of whether they’ve participated in any wrongdoing or even finished their initial training. It would include cooks, secretaries, doctors, and nurses. Depending on the country, it could also include people whose job is observing military practices for the purpose of accountability. The line between military and civilian gets blurred at the edges. So even if you think every last arms carrying soldier deserves death (which I don’t), that’s a lot of collateral damage.

And it’s entire military? No.

Not the entire military. Severing the head of the monster should suffice.

Yes.
Even if you are not “actively” fighting, the role you play is still facilitating the invasion.
Whether you organise equipment, feed the soldiers, or treat wounded soldiers so they can get back into the fight, it is all in support of the greater act.
In 2015, Oskar Gröning, a “mere accountant” at Auschwitz, was found guilty of accessory to murder in 300.000 cases.
You do not have to have literal blood on your hands, to have blood on your hands.

Sure. And all of the politicians, lobbyists, and corporations involved in bringing about the invasion.

I would like a switch instead of a button.

Low level infantrymen and sailors are youngsters – in their teens and twenties. I’m not sure that they’re sensible targets. They are peoples’ sons and daughters,, husbands and wives.

I would have much less of a problem with decapitating the leaders and strategists.

BUT – beware of what you wish for. Their successors may be even worse.

Depends, if say Canada invaded the states, well then break out the popcorn…