|Battle of Canaileus Prime|
The Battle of Canaileus Prime took place in 2298. The United Federation of Planets and Myhr'an fought over Canaileus Prime. The Federation won this conflict, leading to a 70-odd year period of non-contact between the two powers. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part One")
The Battle of Canaileus Prime led to a tense, nearly non-existent relationship between the Federation and Myhr'an. Although Starfleet suspected the Myhr'an of attempting to attack Canaileus Prime on their next expansionist phase, the threat did not materialize.
However, in 2372, in a second wave of Myhr'an expansionism after the Battle of Canaileus Prime, the Myhr'an attempted to not only destroy Canaileus Prime with their planet killer but also retake the planet by landing massive ground forces there later in that same year. The Federation fought them off in both instances. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part Two", "Oh, What a Lovely War")