Half Life 1 had a lot of action, while HL2 didn't. It's episodes had more action that it's self. HL2 could've been more diverse seeing as there's a massive alien empire ruling Earth.
How did the Combine take over Earth in only seven hours?
what you guys dont know is that, that massive portal you saw in Ep2, happened all over Earth, not just in a single spot. The Combine was stuffting out military forces like you can't believe out of these portals. Not just that, we were also trying to eradicate the vortigaunts and the other Xen creatures at the time, so the combine were pretty unexpected since they're not just a bunch of animals and have brains.
The combine APC is like a tank, only it moves 6 times as fast. The striders are destructive monsters that can take a beating. Those flying synth things that shoot boolats, can forget up infantry pretty easily. Their kind of projectile they use isn't a bullet, it's an energy burst, dark energy, it will burn right through a lot of our material, so building a tnak out of the hardest material we have may do a minor punch against the combine, but it wont be long until it's blown up.
The G-Man intervenes a lot on Gordon's journey, making Gordon's job somewhat easier. Also, the reason Gordon can survive because the HEV suit uses efficient sustained energy on his suit, when you pick up batteries that recharges it, but it's also made out of some pretty tough elements explaining why he can be shot a lot without any juice in his suit.
Half-Life 2 can be fully explained, just go to the wiki, I used to go there all the time during school.
