Well, since Congress was not in session and available to act on the opportunity, Jefferson, in order to allow Congress to later exercise its discretion, had to step in and secure the territory. Congress could always have disposed of the land later, had it wanted. But the same people who would never object to the purchase of the Alaska territory have never stopped bitching about it because, in their opinion, the annexation expanded the wrong section of the country.
Have you read Thomas Fleming’s book or The Great Gamble by Charles Cerami. or is there one you prefer?