What is the most monumental decision made while drunk?

You may enjoy this article on "vodka diplomacy." Probably the most historically important moment the author cites comes to us in a letter from Sir Alec Cadogan, Permanent Under-Secretary at the Foreign Office, who accompanied Churchill on a visit to Stalin in 1942: basically, Stalin gave his guests all the alcohol they could hold, and they all had a fine, merry evening together. Because Stalin and Churchill got along so well, Cadogan argued that "conditions have been established in which messages exchanged between the two will mean twice as much, or more, than they did before." That is to say: the bacchanalian evening directly facilitated the sealing of the Grand Alliance of WWII in which Churchill was able to act to some degree as a go-between with FDR and Stalin.

Another contender: John Wilkes Booth reportedly "reeked of brandy" as he ran from the Ford Theatre after assassinating Lincoln. He had of course made the decision some time earlier and just needed a bit of liquid courage.