The only similarity is the title and the guy in the orange hazmat suit. Change the color of the suit and the guy's face and it would be a completely different game.
lol, hl1 had no real story.
you were a dude wearing power armor, you never talk, and then stuff goes wrong and everything tries to kill you. hl2 introduced a proper plot.
The story for Half-Life was that you were working on a project involving interdimensional portals. The experiment goes wrong and creatures from the other dimension begin pouring into the Black Mesa facility. Then the military shows up to destroy all evidence of the mishap and prevent the aliens from escaping any further. Gordon has to make his way through the facility, fighting aliens and soldiers along the way, to the eponymous Lambda Complex. From there, surviving scientists teleport Gordon to the alien dimension of "Xen", with the mission to destroy the massive Nihilanth which is keeping the portal open.
Keep in mind this game was released only a few years after Doom, whose story was "Demons are invading the Earth, you must fight your way through hell to destroy a goat skull which is keeping the portal open", and yet it's still a more entertaining story than all these military shooters nowadays. Come to think of it, Half-Life's sounds a little similar but I digress.
As ridiculous as Half-Life's plot was was, it was way better than Half-Life 2's, in which a bunch of ridiculously humanoid aliens with no relation to the first game show up on Earth and manage to conquer the entire planet in seven hours, despite possessing such horrible equipment that the remaining troops stationed on earth end up using human weapons anyway. Gordon Freeman, some random-ass scientist, must assist a ragtag band of rebels in doing what the organized and disciplined militaries of the planet couldn't, and overthrow an oppressive interdimensional empire. Also there are headcrabs everywhere and antlions for some reason and I just stopped halfway through because the game couldn't decide if it wanted to be a military shooter, a "bug war" shooter, a zombie shooter, or an alien shooter.
HL2's plot could be simplified to
you were a dude wearing power armor, you never talk, and then stuff goes wrong and everything tries to kill you.
just as easily as the first one, with the sole exception that stuff already went wrong before the game started.
because it's incredibly obvious that they're humans wearing suits, because you see a character take off the mask not even five minutes into the game?
Humans can join the Civil Protection, they're just cops. The combine military forces are alien.