The Gorn Hegemony has had a challenging relationship with the Federation ever since first contact in 2267. Much of the tension has stemmed from the Hegemony's claim over the Cestus system, which lies within Federation space two light years across their border. Twice, once in 2267 and then again in 2379, the Gorn have attempted to annex the system. The first time, they were repelled by the Metrons after Captain James T. Kirk defeated a Gorn captain in battle. The second time, a Gorn armada successfully took the system and declared war on the Federation. The Gorn Crisis that followed took many lives on both sides before a combined fleet from the Fourth Fleet's Task Forces 63, 72 and 93 drove them all the way back to their homeworld.
The war did significant damage to the Gorn war machine. Rather than ending the tension, it only drove the Gorn to want to strike back even harder someday. Not only had the Federation bested them, but they also struck deep into the Gorn consciousness when, after the Gorn Hegemony submitted its surrender, one of Starfleet's flagships fired a torpedo to destroy their capital city of Gornar. Now, almost ten years since the last shots were fired, they're ready to strike back.
Invasion of Cestus and Canterra
On July 2, 2388, a series of automated listening posts along the Federation's border with the Gorn Hegemony went dark. A few hours later, massive Gorn fleets dropped out of warp in the Canterra and Cestus systems, laying waste to Starfleet ships and orbital facilities and bombarding the colonies below. Simultaneously, Xavier Fleet Yards and several other assets nearby were mercilessly attacked in a series of hit-and-run raids, crippling Starfleet's infrastructure in the region.
Reeling from its losses throughout the sector, Starfleet was unable to organize an immediate response. By the time such an option was on the table, it was too late. The Gorn had fortified their positions within the occupied territories, and the Federation Council bade Starfleet to move cautiously, the Gorn Crisis of 2379 still fresh in the minds of many.