Zoop wrote: The Empire is supposed to be gone, but yet, here is the First Order, and they managed to build an even more ridiculous superweapon, without anyone noticing? What was the new republic doing all this time? And then they manage to make an entire fleet of star destroyers with the same level of power?
The newer novels & comics expand on all this.
The First Order is the remnants of the Empire that escaped into the unknown regions and had 30 years to do nothing but rebuild itself.
The New Republic negotiated with emissaries of sorts from them and they made false promises to not make any new Star Destroyers.
Leia called bullshit and tried to sound the alarm but the politicians wanted to stay in denial and refused her and so Leia created the Resistance to free what world's the First Order still controlled.
Starkiller Base was presumably sculpted while still in the Unknown Regions hence why the New Republic never discovered it. Problem is the film never shows it going into hypespace and when it's used on the main planets of the New Republic it looks like it's really close to them.
The one intelligent thing they wrote with Starkiller is them fueling it by draining energy from Stars instead of using crystals.
The Emperors Fleet was built separately by him commanding a mostly droid army and he also had 30 years to mostly work on just that under even more secrecy then the First Order had.
To be clear I'm not happy about them beating the superweapon cliche into the ground either. And especially with them being so similar to each other. In the old canon the Sun Crusher & World Devastators were all at least unique designs & systems. In the film's we have had 4 planet destroying generic laser guns in a row.