Federation-Klingon War of 2405-2410

The fifth Federation-Klingon War took place between 2405 and 2410. Initially a dispute over the extent of Undine infiltration of the Alpha Quadrant that escalated into a land grab by Chancellor J'mpok of the Klingon Empire, it ground to a halt by mid-2409 in the face of mounting threats from outside the region occupied by the two superpowers, and was officially ended early the next year. 

Backdrop to the War
The rise of the war began with the Klingon-Gorn War. While the Klingons were slowly pushing the Gorn back and taking their worlds, it wasn't until J'mpok took over from Martok did the Klingons conquest of the lizard race kick into full gear. This was made especially clear when Ja'rod, son of Torg discovered that the Undine had inserted themselves far deeper into Gorn Hegemony than believed. J'mpok ordered the invasion of the Gorn homeworld, which the Federation condemned. In retaliation, the Klingons pulled themselves out of the Khitomer Accords and threw out Federation citizens from the Klingon worlds. The Klingon invasion culminated with the revelation of Undine within the Gorn ruling faction and their swift execution. 

Opening Shots
The first strike came when the Klingons attempted to invoke ancient rights to the Hromi Cluster, demanding all Federation citizens and assets removed in three months. When the Federation refused, the Klingons started their attack, invading the Korvat system to forcibly eject them. Starfleet retaliated. As the war raged on, the Federation continued to try to stop the Klingons' advances through peace talks, but J'mpok refused. As the war raged on, Starfleet's numbers started to dwindle, leading to a revision of the rules and the reactivation of many old classes. 

B'vat's War
Though the Federation was still willing to negotiate peace and some Klingons were willing to listen, one Klingon wanted to drive things into an endless sea of blood and chaos: Ambassador B'vat. Convinced that the Klingon Empire would stagnate without a target to fight, he sought to drive the war further and further along. His first strike would be to interrupt peace talks on Regulus IV, seeking to kidnap Miral Paris in the process. A Starfleet captain helped drive away B'vat, but ruining the peace talks due to the Klingon's interruption. Soon after, an Orion named Marta revealed information on a secret weapons factory within the Briar Patch, leading to a Starfleet captain to confront it and destroy it, though B'vat boasted that he already had much more deadlier weapons than that. 

This was proven true after discovering Klingons and Gorn stealing various medical supplies, intending on restarting the augmentation process. Starfleet was able to stop this and bring Amar Singh to justice. At the urging of the disgraced Klingon warrior K'Valk, Starfleet attacked Imaga after B'vat's ultimate weapon, a Doomsday Machine, was discovered. Through K'Valk, they learned that B'vat plotted to use the Doomsday Machine to bring about his dreamed endless war by unleashing it on an inhabited world, forcing the Federation to attack the Klingon Empire as revenge. K'Valk and a small team of Starfleet personnel were able to infiltrate B'vat's base and sabotage B'vat's workings, turning the Doomsday Machine loose. K'Valk sacrificed himself to damage the Doomsday Machine while the personnel used a special new torpedo, the Hargh'peng, to actively destroy the weapon. 

Growing desperate, B'vat's men attacked the Federation ship USS Kirk (NCC-91277), kidnapping Miral and headed for the planet Gateway. B'vat dragged Miral through the Guardian of Forever, a Starfleet captain and his crew following suit, both ending up in the year 2270. There, Miral was rescued as B'vat was finally killed, urged by his younger self to grant him the death he deserved, though not before Miral's blood was used to help restore Klingons's lost ridged foreheads. 

To The End
After B'vat's death, the war ground out to a stalemate as other threats came about: the return of the Borg, the Reman uprising against the Tal Shiar, the Jem'Hadar invasion of Deep Space 9 and the resurgence of the Undine. In late 2409, the Romulan Republic discovered an Iconian Gateway in their world of New Romulus, seeking to activate it. Doing so lead to a series of events that revealed the Solanae Dyson Sphere and encountered the Voth. Seeking to stop the Voth from capturing the Omega particles inside, an alliance lead by Rear Admiral Tuvok disabled the Sphere's jumping capabilities, but opened a second Gateway, leading to the Jenolan Dyson Sphere. War nearly broke out again as the Romulan Republic and the Klingon Empire tried to claim this Sphere, something the Federation had already done years prior. At Tuvok's urging, a summit was held on the Jenolan Sphere to discuss the Spheres and the reappearance of the Undine. 

In 2410, the summit was held with Ambassador S'taass accusing the Federation of only taking interest in this because something of their own was being threatened. However, Captain Va'Kel Shon of the USS Enterprise (NCC-1701-F) changed things by admitting that the Federation made a terrible mistake in not believing the Klingon Empire. Things ground to a halt when a massive Undine force invaded the Spheres and raced towards Sol. The three factions made their way to Earth, but not before the Undine heavily damaged Earth Spacedock. A mental attack on Tuvok by "Eric Cooper" would lead them to realize the Undine's true target was actually Qo'nos. The remaining Starfleet forces not damaged in the attack aided the Klingons and the Romulans in the rescue, watching in horror as the Undine unleashed their own planet killer. Shon destroyed the weapon by ramming it with the Enterprise's USS Aquarius light escort.

Finally united with a common enemy and shown the error of their ways, the Federation and the Klingon Empire would bring a cease-fire to their war. However, before they could, an Iconian appeared before them, murdering the entire High Council before leaving. This would only solidify the restored bond between the two superpowers as the war came to an end.