The Webster–Ashburton Treaty is a decision the United Kingdom can enact. It transfers control of Caribou, an American core in Maine, to the United States of America. In return, the UK receives +21 prestige, −2 infamy, and +100 relationship with the US. To enact the decision, the UK must be at peace with the United States and, of course, Great Britain (or one of its puppets) must actually own Caribou. The AI will take the decision so unless it has less than 0.2 infamy. Furthermore Canada loses its cores on Caribou and on Bangor, the province south of Caribou.
The decision is available from the start of the game.
Caribou will be ceded to USA even if New England exists, and can be ceded to the USA no matter what the USA's territory looks like - which means it is possible to give Caribou to the USA as an exclave if it has lost the rest of New England.
Because Caribou is such an insignificant province, only containing 9k POPs with a timber RGO (plenty of which are scattered throughout the rest of Canada), it is recommended to take the decision as the UK once you have more than 2 infamy. However, if a Britin has colonised other American Cores (in Washington, Idaho, or Oregon), then the decision will not remove all American cores on British territory, and as such is less useful - and it might be worth not enacting the decision in order to retain Canadian cores in New England.
Illogically, the decision is placed in the USA's ("ACW") decision file instead Britain's ("ENG") in the decisions folder of the game files.
Following the events of the (undeclared) Aroostook War, The Webster-Ashburton Treaty of 1842, between the United Kingdom and The United States of America, delimited with finality the borders between Maine and New Brunswick, regulated the shared use of the Great Lakes, reaffirmed the 49th parallel border and called for a mutual commitment toward ending the Atlantic slave trade.