Sorry I don't recall much of it because I was a little boy back then but I remember watching this awesome cartoon about a squad of soldiers who were supposed to fight alien invaders. Also, they were all dressed in black suits(not MIB) and the name of their spec. force consisted of 4 or 5 words that formed a code name ( something like s.w.a.t-spec. weapons and tactics). If anyone knows this one please let me know! I just can't find it anywhere. Thank you!

Could it have been S.H.I.E.L.D like in the Avengers?

No. It's not shield. I only remember a fragment of one episode when they had to hold back a massive zombie attack and prevent them from entering a base or headquarters of some sort.