The combine were much, much stronger than their occupying force was. Those soldiers you fight didn’t exist until after the takeover - what they attacked with was all those synths (so like, thousands of striders), the advisors, and some unseen combine non-human soldiers. But, think about the advisors - seriously, we have killed NONE of them in any of the HL games. Even DOG, a robot that could go toe to toe with a strider, only injured one at best. These things alone could just /stroll/ through our armies, sucking each soldier’s brain out one at a time. I believe it was a plot point in Episode 2 to make DAMN sure the portal was closed because otherwise the reinforcements would stomp all over us, or even possibly say screw it and destroy the planet.
Also, again, Xen works because they really are infinite. The borderworld has species pouring in constantly, either on the run or simply lost. They end up crossing over. Furthermore, as shown in Interloper, alien grunts are mass produced in giant organic factories. It’s like a zombie apocalypse - no matter how weak the enemy might be, you have limited men and ammo. Sooner or later, your tank armour is gonna break down from constant impact, you’re gonna run out of ammo, and you’re gonna get tired. That’s when you lose.
That’s what happened to the HECU, they just couldn’t keep fighting. Oh, and then the indestructible aircraft arrived to screw them over further, and the damn hard to kill gargantuas.
Oh, and bear in mind the Xenians were plaguing earth at the same time the combine attacked - BM’s destruction was a failure as more portals started to appear through which they attacked us.