200 Years ago Caldari Ultranationalism collided with Gallente Liberalism, Gallente did not appreciate Caldari treatment of thier own people even though the caldari where perfectly fine with it (and to this day still are, concepts of indentured servitude, generational employment and megacorporations are still at the heart of caldari society)
This eventually lead to aggression in politics, then terrorism, then war. Caldari Prime located in the same system as Gallente Prime the much smaller nation (as the federation was interstellar) had nearly no chance to win. Two things changed that:
https://www.youtube.com/watch?v=yXWf-l71MY4and
http://community.eveonline.com/backstory/short-stories/the-jovian-wet-grave/120 years passed after the end of the war and the formation of concord to create a relative peace
In recent times, A Caldari criminal military thug known as Tibus Heth, leader of the paramilitary group the Templis Dragonairs, managed to grab the reigns of the Caldari megacorp board which for the longest time held equal power without a figure head. Self titled Executor Tibus set about retaliating against the gallente for betraying a peace summit
https://www.youtube.com/watch?v=uSHdIsIOrsc&list=PLQvKSs1k6DLM_A7Y5dbK5uH5kOwNDALK7&index=29After the incident Tibus Heth managed to get a capital fleet to warp over caldari prime to get past the hole in the cyno jammer defenses and parked a caldari titan over orbit. Since the evacuation of caldari prime the gallente have colonized the planet and with the titan in orbit basically held the entire planet's population hostage.
Gallente trapped on the planet where forced to live in hovels and camps and the caldari came back to repatriate the planet. Any attempts to land, or even bridge in capital ships would have vaporized the entire planet's population. The entire ordeal forced the previous gallente president Jaques Frotaine to resign and Joshua Rhoden replaced him as federation president.
After nearly 5 years of occupation, Heath's mental conditioning was breaking down, his thug rule over the caldari citizens was taking its toll disenfranchising its citizens of the caldari way including members of his own corp Kalaklonia. This was worsened when Tibus Heath was shot by an unknown immortal soldier. Tibus' delusions, fear, and hatred caused him to act hastily and make threatening moves. He first turned against the immortal soldiers of Osomon Surveillance Group. Lashed out at other corps branding them as traitors during a humanitarian crisis and thinking against his wishes, and eventually preparing to take a full capital fleet into Luminare with every intent of possibly destroying the gallente threat for good.
A culmination of opportune movement, politics, and luck lead to the battle over Caldari prime. The Admiral assigned to the caldari titan, refused orders to doomsday the planet. This refusal would lead to her dishonorable permanent death at the very hands of Tibus Heth (she was a capsuleer) but her sacrifice nearly saved everyone on the planet both Caldari and Gallente.
Neither state nor federation could secure a clear victory; there was still Caldari immortal soldiers still bitter over tibus' hunt on them and refused to fight, while the outnumbered gallente couldn't secure the victories they needed despite the chaos on the other side.
Mordu's Legion along with Ishukone broker peace with the Gallente over the planet and so far mordue has maintained status quo.
Tibus Heth however has yet to answer for his crimes against the state. Shortly after the Battle for Caldari Prime Tibus Heth was labeled a traitor, terrorist, his position of executor dissolved and abolished, revoked of all stocks and assets frozen, voted off the board for KK, held an entire station hostage and then escaped. His last known whereabouts are unknown his escape ship was found and it looked like it was boarded.