The portal side of the story is all a lot more light hearted, actually laugh out loud funny in lots of places. The combine in Half Life 2 are actually sort of the old parts of humanity and other worlds that the combine have collectively absorbed.
They assimilate everything, from languages to resources like the oceans to the things that live their themselves. They go from planet to planet, taking every resource they can and making it their own, then using that planet as a base to launch new attacks. The Combine are like this mix of the flood and the borg, who are slowly conquering the entire universe. The combine aren't a singular race or entity of creatures, they're genetically engineered 'things' that they assimilate like the borg. The stuff about the combine in particular is pretty horrifying. Humanity has already lost in Half Life 2, we're being extinguished essentially. The world in Half Life 2 is really fucked, they're right in the middle of teraforming it in an awful way. It's called the 7-hour war, because that's how long it took for humanity to surrender. There is actually an enormous backstory to both Half Life and Portal, all told entirely through secondary background items that are super missable if you don't pay attention. Go read news paper clippings and listen to radios as they chatter and letters left on desks and such. You need to go replay Half Life 2 and pay attention to the story told through the surrounding areas, lol.
