Was just on reddit where someone asked why Insomnia didn't have a huge army... I tried to give my take from what I understood from the lore.
Having watched KG like a dozillian times... from my understanding of the general lore, the Empire purposely left Insomnia by giving them a false sense of tranquility. Why?
Insomnia has been protected by the barrier wall for a long time, in which the Empire had failed to bring down. And the person holding the wall in place was Regis along with the crystal, who in his prime years was still very strong.
The only way the Empire was going to get what they want, was to kill the source. So they waited for one of the biggest long con ever.
When one is inside a boxed wall, they are given a false sense of security that they are safe, making them feeble and weak over time. Though Regis may have been prepared, the citizens of Lucis were drunk on prosperity. As in the early scenes of KG, you could see people acting like hooligans and drinking in broad daylight while Nyx, returning from their mission watches in disgust. The Crownsguard Captain also boasted the ''peace'' under the wall, that as long as the wall stands that they were safe.
From this mindset, the people of Insomnia became fools weakened by their own ignorance. Which is exactly what I think the Empire wanted. Weaken the enemy and create holes everywhere.
Also, most likely the Empire knew of the relationship of Drautos and Glauca. So they used him as well, letting him think that he was playing by his own game so that he would kill Regis. And since it was Drautos who commanded Kingsglaive, being one of the founding members, it was a group that was always full of doubt.
And at the time of the Invasion, Regis was at his weakest, aging faster day by day, losing his powers slowly. This was the perfect time to strike before a new young King was appointed. This is probably why they set up the wedding, so Noctis wouldn't be in the city.
Regis could have prepared differently and instead had Noctis stay and take role as the new King, but Noctis too was weak by his own ignorance and was not yet ready. So there was a chance he would have been killed also.
As the Empire left Insomnia to crumble under their own peaceful prosperity, they offered a true alliance of peace, sealing the deal making the people even more weaker. The citizens were blinded even more and the Glaives were blinded by anger. So the nation was severely weakened.
Some ask why the Glaives consisted of immigrants, but it was exactly that they were immigrants that they were needed. This goes back to the citizens of Insomnia being drunk on ignorance of prosperity, they had no resolve to fight. However, the immigrants did, they lost their homes and their people to the Empire, they were not fond of peace, everyday was war for them, so they had the resolve to fight. But at the same time, they were outsiders making them weak with doubt. Unlike Magitek soldiers and daemons, who are immune to emotion, Insomnia was swayed by ignorance.
Regis, probably knew what was coming, for the most part at least. But instead of preparing to defend against it and protect the nation, he was willing to sacrifice it all to save Noctis, so Noctis could save the world.
Adding on to this, the perfect example appears in ''The Walking Dead'' series in the Alexandria Colony. A community protected by a wall, experiencing a long lasting peace, they were all weak with no real experience of the ''real'' world. They were also blind from their own ignorance, which even got most of them killed. The ones who survived were outsiders who fought to survive everyday. But when you have that gap between two largely different groups, there are many holes, in which ''others'' used to get inside the walls.