He used one of his agents to bribe Bernard Loredo, so he could assist Kaedwen during invasion on Temeria.
At the beginning of act 2 of the game he meets with Prince Stennis to negotiate conditions of the end of their war. However the negotiations fail due to a curse that unleashed a demonic fog. It also postponded his invasion. Geralt of Rivia helps to break the curse him or his opponents by killing Draug. Depending on player's choice King Henselt can conquer Pontar Valley or lose the war. In the former scenario player can also allow Vernon Roche to kill him, in revenge for the massacre of the Blue Stripes.
In Witcher 3: Wild Hunt if Henselt survives the previous game, then is killed by King Radovid during conquer of Kaedwen.
The Witcher 2: Assassin of Kings
The Witcher 3: Wild Hunt