I watched this movie or TV show episode in early 2000s.
It's about aliens who come to invade Earth, but they look human and speak like humans. When they talk to each other though, they do it by croaking and we can't understand what they are saying.
Eventually humans are about to lose the war and aliens are about to win. But humans built some kind of a doomsday weapon, which would damage Earth and make it unsuitable for alien use. The scene I remember is in the final bunker holdout of humans, where they enabled the countdown on this doomsday weapon.
The alien "army", which looks just like normal human infantry, are fighting their way into the compound. The "Earth president" (I don't know his exact title, but I believe he was from France) is inside the bunker, and he claims that he found a diplomatic solution with the aliens and wants to shut down the doomsday countdown. But the other remaining humans are holding the president at gunpoint because they don't believe him or don't believe in his solution, while the countdown continues.
Then something happens (on the console?) that requires the attention of all the experienced humans, so they give the gun to the youngest most naive guy and tell him to keep it on the president. The president continues pleading, and says something like, listen to reason, I can guarantee your safety.
Finally with only 3 seconds or so left on the doomsday countdown, the young guy caves in and gives the gun to the president. Then in just a couple of seconds, the president shoots and kills the several experienced humans, then shoots the doomsday device itself and the countdown stops.
In the next few seconds, the door is broken down by the alien army, and the aliens walk in wearing army fatigues and start speaking in croaking voices. As the naive young guy watches, the "president" speaks back to the aliens in a croaking voice. I believe the president then shoots and kills the young guy.
Edit: Okay, I remember some additional details. This whole scene is a vision of the future - what would happen, if the invasion is not stopped in the here and now. I think there is a protagonist fighting to prevent the aliens from infiltrating humanity in the present. And every now and then, we are shown sequential scenes from the future. Finally ending in this complete defeat scenario for the humans, at which point we know that if the hero doesn't succeed in the present then all will be lost in the future.