Is the US obligated to protect Alaska from invasion?

Yes, the United States is obligated to protect Alaska from invasion. Alaska is a state of the United States, and as such, the United States government has a responsibility to defend it from attack. This obligation is enshrined in the United States Constitution, which states that the federal government has the power to "provide for the common defence." Additionally, the United States is a signatory to the United Nations Charter, which commits its members to "refrain from the threat or use of force against the territorial integrity or political independence of any state."