good but confusing????????????????

this is an amazing series but really really confusing... I do not understand why the trailer seems like the continuing of the last part of ep1 part 3 when Its pretty much the trailer of the whole series and should be what comes first. Its amazingly confusing how america was the one military to begin ww3 and not any other enemy military... and you could explain if the trailer is the trailer of ep1 series or some other episode after that?