Porre's existence was a very quiet one compared to Truce and the Kingdom of Guardia; other than a few scattered events between Middle Ages and the Present, not much is known about the history of this location. What is known is that in the Middle Ages, Porre was cut off from the rest of the continent during the war with the Mystics. The Zenan Bridge that linked Porre and Dorino to Truce and Guardia Castle was dismantled to prevent further incursion from Mystic forces.
There was a legend that the one to defeat Magus would wear the Hero's Badge; a young boy named Tata was expected to be the prophesied hero reborn, but when he revealed that he had just found the medal in Denadoro Mountains and not by some heroic means, he happily gave the medal to Crono, who in turn gave it to Frog.
Since Frog lives where the Mayor's Manor exists in the Present, it is likely that following his death, the Mayor built the manor there. After these events, no further problems occur in the original timeline (the timeline which leads up to the Day of Lavos) or the final timeline in the game (depending on your viewpoint determines when the party defeated Lavos). No further action is taken regarding Porre.
After charging Marle's Pendant with the Mammon Machine in Zeal, several items that were unable to be reached before could be received. In Sealed Chests, Black Vests and White Vests can be found during the Middle Ages. Black Plate and White Plate are found in the same chests during the Present
According to the creator of the Chrono Series, Masato Kato, an influence from "outside" of the timeline helped Porre gain immense power and become the world power that it is seen as in Chrono Cross. In a new dungeon added in the Nintendo DS remake of Chrono Trigger, it is revealed that Dalton is the one who intervenes, in an attempt to exact revenge on Crono. In Dalton's own words, "Just you wait! I'll raise the greatest army the world has ever seen in Porre, and use it to wipe your pitiful little kingdom off the map!"
It is unknown how Dalton was able to achieve this. We do know for a fact that after the Time Crash which occurred during the events of Chrono Cross, people from Porre settled on the continent of El Nido (which was itself an area terraformed by Chronopolis, brought back by the Time Crash).
During events not shown in any game, Dalton apparently traveled to the present and helped Porre amass an army, of which General Viper was a member (between 1003 and 1005). In 1005, the army attacked Guardia; due to the inability of the player in Chrono Cross to travel outside of the El Nido archipelago, it remains unclear exactly what condition the Kingdom of Guardia was left in. Speculation grew as to whether King Guardia or Marle and Crono (shown at the end of the PlayStation release during their wedding) were killed; though based on Lucca's letter dating from after the attack, she refers to them as still alive. If this is the case, it is unknown how they evaded Porre's army. During the attack, the Masamune, long in the vaults of Guardia, was stolen.
After successfully defeating Guardia, Porre turned its eyes towards El Nido. In-Home World, the Acacia Dragoons had been killed prior; this left the continent open to invasion in 1017 A.D. In Another World, the Dragoons had survived; it wasn't until Lynx got them out of the way in 1020 A.D. that Porre invaded the continent. Much of the reasoning for the invasion was due to the search for the Frozen Flame; Norris was in charge of the search before joining Serge.
Escorted by the Porrean Legion, Vera (disguised as an old crone) infiltrates Viper Manor, hoping Serge will lead them to Lynx's treasure, the Frozen Flame. After Kid breaks the Chrono Trigger, Vera and the others arrive to appropriate the sacred mineral. The party flees the Manor and the soldiers scour the nearby forest, searching for the missing thieves. Kid and Gil escape, leaving Serge behind, saying that the troops will not capture him; She and Gil are their primary targets. Probably because Dalton had ordered the search, he was somehow aware of Kid's identity as Schala incarnate and Gil's identity as her brother, Janus, and wanted to, yet again, abuse their magical gifts for his own gain.