Why in the world would the humans want to fight each other

Well that's the whole point of humans we can work together and do amazing things but we rather kill each other instead it's the human way we average a devastating wars every century I mean look at WW2 50-60 million dead (I know the movie takes in a bit changed reality of WW1 date wise) and that would happen in real life too if a alien specis almost wipes us out we would have to put aside our problems and hate and unify as one banner of humanity and fight as one and once it's over start rebuilding 25yrs later we slip back into our old ways

What I described is like a alternate history timeline what if scenario by Harry Turtledove "Worldwar" series which is like 7 books but really good.It basically takes place during the Begining of WW2 and a reptilian race invades all of earth so we all have to stop fighting each other and try to fight as one (although in the series you'll see it's not so easy and how most countries will want the superior technology) the timeline runs from WW2 1941 all the through (if I remember) to the last book into the 2100s.so it covers all the timeline in between with really cool but crazy what if timelines,give it a read the first book is called "Worldwar In The Balance" and follow from there on