Well look, most likely El Nido and Chronopolis have nothing to do with Porre's rise of power, as Chronopolis is the result of a powerful nation Porre. Therefore it would be an impossible paradox if Porre's power came from Chronopolis.
Anyways, either The Frozen Flame, The Masamune, or a powerful person appeared to make Porre strong. Why don't we examine what happened to the Masamune after 600 A.D. Glenn probably put it back in the Denadoro Mountains which are inaccessible in 1000 A.D.
Well we know The Mayor of Porre was into treasures (the sun stone), so maybe when Dalton appeared he forced him to find it. And the guy at the right does look like Dalton in Life Skips a Beat.
Also, Porre soldiers have a similar stance to what Thrashers did. And I believe there was "absolutely" no military in Porre before either Dalton, the frozen flame, or the masamune appeared. I mean, tell me "where" are they? Where is this military in CT? Well I'll tell you where in Guardia Castle that's where. In 600 A.D. soldiers from Choras, Porre, Dorino and Truce all joined the Guardian army.
As for the Porrean Beret, whatever it meant, it sure didn't mean Porre had a military, I'm sure the creators of CT didn't have CC in mind when they made it. It was probably just a beret made in Porre. Also notice Porre has guns in CC. The only people who can make guns are Lucca and Taban. Perhaps Dalton had Taban kidnapped?
Anyways, a few hundred well trained men, Dalton with the Masamune, and possibly the Frozen Flame would be enough to take down Guardia and Crono.
Or maybe, just maybe, the "shadow" at the right of Life Skips a beat is Crono himself. Crono's no weakling, but people like Garai and Dario were killed by the masamune and they were tough. Hmmm, I still don't really think Dalton could've killed Crono, perhaps if Dalton never did appear in 1000 A.D. and simply remains "unknown" but in 12,000 B.C on some hidden island with Queen Zeal.
Hmm, let's take all the theories I just stated but replace "Dalton" with "Crono". Let's say after the Masamune became evil, Crono probably didn't know this, and the king of guardia asks him to find the Masamune to keep for safe keeping (they already have the rainbow shell).
Crono probably searches through Denadoro mountains pulls the Masamune out which Glenn put there 400 some years earlier. Crono goes crazy, and heads to Porre and Porre for some reason (taxes?) wants Guardia dead also, so Crono and these troops march to Guardia and destroy it. I mean, you expect Crono to have the same hairstyle for 5 years? Perhaps the guy on the right is him. But perhaps the finding of the masamune and the downfall of Porre took place some months or years apart.
It says "dissappearance" of the masamune. Well we don't know when it appeared in that era. Or how for that matter. Anyways, Crono possibly had kids with Marle before he went insane with the sword.
This theory could possibly take place in the El Nido/Chronopolis timeline, but we can't say how much El Nido really did effect the Zenan mainland.
Anyways, hope you enjoyed my theories. But I do hope the next Chrono game explains it. I call this the Porre-Crono theory.
P.S. In the Lucca letter she makes absolutely no mention at all of Porre taking down Guardia or even Porre for that matter. So Lucca found Kid in 1004 A.D. She probably wrote the letter in 1004 A.D. also. Obviously it was written sort of like a will. Anyways, she probably lived with Kid until like what? 1013 A.D. around that. There's nothing implying "when" that letter was written, all we know is sometime after 1004 A.D.