Second Federation-Klingon War

The Second Federation-Klingon War was a conflict fought between the forces of the United Federation of Planets and the Klingon Empire from 2289 to 2290. Although the war officially ended in a stalemate, the last, climactic battle of the war at Calder II saw an, albeit costly, Federation victory, and the Federation gained some Klingon-held territory, which they were able to keep per the treaty, signed on Calder II.

Open War
The actual war began when Chancellor Gorkon, urged on by the members of the Klingon High Council, issued an ultimatum demanding the Federation's complete withdrawal from the Archanis sector within ten days.

The Federation refused and the Klingons began a full-scale attack on the Federation, committing all available military resources to the war. In the ensuing fighting, the Federation proved almost completely unprepared for a sustained conflict on the 'secure' Klingon border, several dozen starships and border worlds were attacked during the fighting across a wide swath of territory from Archanis all the way to the Sector 654. After some time however, Starfleet managed to slow down the Klingon advance; furthermore, it was implied by Worf, the leading member of the House of Mogh, that the Federation would eventually manage to rally and push back the Klingon advance with steadily increasing proportions of losses to the Klingon Defense fleet, urgin the Chancellor to seek peace.

In 2290, a Federation task force was ordered to attack and secure the Klingon occupied world of Calder II, which was rich in dilithium crystals. The Klingons had been leaked intelligence about this, and they moved their entire fleet to defend the planet. However, Captain Jerry Mcarthur had word that the attack was a trap, and, disobeying orders, rushed into the battle, which had been going badly for the Federation due to a prototype Vorcha class vessel present. With no other choice, McArthur rammed his heavily damaged flagship, the USS Gemini-A, into the IKS Vorcha, destroying it. After the loss of their most important vessel resulted in a Federation victory, Gorkon convinced the Klingon High Council to call for a cease-fire. Two weeks later, the Calder Treaty was signed, ending the conflict.