It might be "King David's Spaceship" by Jerry Pournelle.
The story has a couple of sections that you might have merged in to one.
There was an empire among the stars. It collapsed, and the individual planets got along as best they could. Some died out completely, some lost all use of technology and had to rebuild, and some retained enough technology to regain star travel after a relatively short time.
King David's planet is one that didn't do so well. A more advanced planet is intent on rebuilding the empire, and has come to force the people of King David's world to unite under one government and be admitted to the new empire.
The bit you mentioned about s "he watched the picture of his family melt, and he along with it" reminds me of one of the battles that took place. Colonel McKinney is one of the better officers of a smaller kingdom on the planet. He and his men actually managed to shoot down one of the (laser armed) imperial landing ships in battle. This causes the local empire commander to have another landing ship destroy the whole lot of them in a second attack using the laser directly on the men (rather than on the cannons or other equipment.) McKinney survives, but his soon to be wife was in the area that got blasted. Much later, as he is telling the story, he uses a phrase very much like what you wrote.
The other part of the story involves the imperial traders taking a group from King David's planet to another place that was even more backwards.
McKinney is chosen to head the group. They are actually trying to get to the remains of an old empire library - hoping to learn enough to build their own spaceship. The new empire takes in planets with spaceships under better conditions than those without.
McKinney does indeed fight (and win) a war under medieval conditions to get access to the library.
Might not be your story, but could be given the lack of details.