As I mentioned above, there is nothing to explain what the war is about. There's just a New Order, which is evil, and uses Imperial hardware, and they are opposed by a Resistance which uses X-wings and is led by Leia, but where each side comes from or who is ostensibly in charge is completely up in the air. There is mention of a Senate and a Republic, and allusions to a government which supports the Resistance, but no mention of why a government doesn't have a military, which is pretty much the sine qua non of a "government", or if they do, what that military is actually doing, while the Resistance undertakes to confront the New Order. BUT they didn't mention taxes or trade or diplomatic missions in the first act, so at least the movie isn't RUINED!!!!
With most people just watching the movies, and not likely to read all the new books released recently / play the various games, I'm a believer that the movies should be self explanatory without reliance on the other material.
Unfortunately, I get the impression that here there is a bit that is perhaps explained in the books, but perhaps they thought it wasn't required in the movie?
My vague understanding of the situation from what I have read of synopses of books is that:
After ROTJ, the Empire was still around and fighting to hold on, there was a big battle at Jakku, where the Empire was decisively defeated, and so the Galaxy was then split in two - the remnants of the Empire and the new Republic.
The Empire eventually became the New Order, and a resistance against the New Order within the New Order territory formed, with covert support from the Republic. So the Republic in theory had a Navy / Army (and I believe this was mentioned in the movie), but wasn't at war with the New Order so not involved in fights.
I also believe that the New Order / Empire were firm believers in strength in Order / dictatorship / military rule, vs the potential issues in democracy.