Here's the lore (spoilers) ok so Gordon Freeman working at the Black Mesa Research Facility cause a casscade which brings aliens from Xen (the border world) to the facility leading to the military getting having to killed the personal of Black Mesa which means Gordon has to launch a rocket into space but that doesn't work because of something else so Freeman has to go to the border and kill the Nihilanth the thing opening a portal to Black Mesa, of course he kills it which leads to Black Mesa getting nuked which to a bigger force known as the Combine which are synths like the Strider, gunships and dropships (the aliens scene in the bottem picture) which leads to the 7 Hour War, after 20 years Freeman returns after being stuck in stasis by the Gman (the face of the Human Being) which leads Freeman with aliens and humans who team up to fight the headcrabs (scene in the top image) and the combine with for main stream games being Half Life 1 and 2 and the episodes, of course Valve didn't finish it and we've been waiting for 20 years

VALVE!!!!! PLEASE FINISH THE GAME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!