Corucant isn't one of them. The New Republic set up in a different system called the Hosnian system (it's a new one).
They were armed, because the New Republic was the Rebellion, pretty much after the second Death Star was destroyed they changed the name, because they were no longer simply a rebellion.
You did watch the film, right? They were killed by Kylo and his knights. Han says this in the movie! It's also what you see in the vision and is why Luke is in self imposed exile.
The Resistance told the New Republic of these weapons but they ignored it or were not sure if it was true. Remember, like I said, the New Republic army was cut dramatically and spread all across the galaxy as well.
Like I said, this sort of stuff is in the books and is explained in great detail there, way more than I am doing, this is just a brief summation. If it was in the film it probably would've destroyed the pace and tone.
If you've any other questions, I'll answer them later, as I'm off to a couple of lectures (yawn).