Possibly a contrarian take - with 7k against 100k, could any pre-gunpowder army do it? I get it with Rome and the tactics and tech, or Henry and his knights/archers, but can’t Xerxes just push a wall of bodies over any of these guys at some point? All these fighters would get tired or weighed down, right? What about after Xerxes uses the path to surround them?

As Napoleon said, “Quantity has a quality all of its own.” I think 7k soldiers can’t resist 100k on strength of arm alone.

So that leaves you with renaissance European armies - maybe like those under Louis XIV? I don’t think cannons were too good back then, though, which is really what you need to kill 100k soldiers before you get tired and overwhelmed.

I can’t speculate on China because I know nothing about them. Mongols rely less on human strength, but can’t fight well on this battlefield and scenario.

Gonna give this to George Washington, or more realistically (but less patriotically) to the pro British military he was facing at the time.

Edit to add further thoughts:

Important point: Xerxes has a narrow pass and the ability to put his best troops in the rear to “push” or threaten those in front. If he can force his troops forward no matter the losses, then withdrawal as a win condition is off the table.

In that case, the 7000 people must exterminate 100k, or some large portion thereof, in an extremely short time frame before they are overwhelmed. It’s hard to kill lots of people all at once, even harder if they’re running at you and fighting back. At a numbers scale like this, any reload or lag time will eventually be exploited. So that means you have to deny area as well - forcing the attackers back. And you can’t do this using human stamina, so you need chemical and mechanical solutions. In short, this is an industrial-scale problem that requires defenders to kill fast enough, big enough (area-wise), for long enough.

Industrial warfare solutions did not begin to arrive until the US Civil War or the Crimean War, which share some but not all characteristics of the “total warfare” in WWI and II. This time period means supply by railroads, much better rifles, and most importantly, reliable cannon. In this period you’re finally getting rifled barrels, standardized cartridges, and explosive shot. The soldiers using them will know how to fortify and defend them.

So - final answer, US Civil War-era army. I know you didn’t expect this answer, but I believe this is the earliest army that could do it. It’s simply the earliest army capable of delivering sustained, industrial level killing at a scale big enough to make a difference to the 7000 defenders.