"From the 3090s to 3128, the Concordat saw a gradual loss of worlds from its sphere of influence. The Capellan Confederation preyed upon its former ally, capturing the worlds of the “Laconis Front”." (FM3145 p185)
"From 3122 to 3137, the Second Highlanders were deployed on the Capellan worlds of Brisbane, Bromhead, and Laconis. As each of these worlds was formerly a part of the Taurian Concordat, they were continually raided by forces from the TDF." (FM3145 p193)
The map on WotRE p62 shows the three worlds are still part of the TauCon after the end of the Victoria War in Jan 3105.
So from the above we can deduce that the CapCon invaded Brisbane, Bromhead, and Laconis after Jan 3105 and before 3122, making it very likely indeed that this is the CapCon aggression VhenRa points out in Kaff's fluff. There aren't any other TauCon worlds conquered by the CapCon during this time frame, nor any mention of worlds conquered, but then re-taken.
It could be argued that Kaff joined the TDF in response to the loss of the three worlds, not that he actually fought in their defense, but the wording in his fluff in ER3145 strongly implies he fought during the actual invasion.
"Kaff entered the Taurian Defense Force even before he was old enough, lying about his age in order to see service against the Capellan encroachment on Taurian space" (ER3145 p112)
Given that Kaff fought the CapCon prior to his 18th birthday, which was when his two year conscription in the TDF should have begun by TauCon law, then the date range for at least one invasion, if not all three, can be fine tuned to between Jan 3105 and sometime in 3107, depending upon which month Kaff was born.
Arguably, Kaff could have signed up due to CapCon raiding mentioned in FM3145, but the FedSuns were also raiding, so if it was just raiding that spurred him into action, then why specifically mention the CapCon? The CapCon invasion of Brisbane, Bromhead, and Laconis is a much more likely motivator.