First of all, sorry for english errors
I was not able to find anywhere a answer to this. Why the U.S. annexed the CSA? I mean, the south started the war, but from what i've read the constitution allowed states to seced at that time so, why the U.S. annexed them?
There's always more that can be said on this matter, but while your waiting you might appreciate our section of the FAQ on the Civil War and answers like:
Did the framers of the U.S. Constitution discuss whether it should be possible for states to secede? with an answer by /u/turtleeatingalderman.
Or this answer from /u/secessionisillegal: If the US seceded then why did Lincoln say the CSA was illegal? Wouldn't people have thought this was a bit strange and hypocritical?