Why not?
USA is so weak and coward chicken.
Alaska is originally Russian sovereignty for a very long time in history, and Americans only spent tiny negligible amount of money to Purchase Alaska from Russia. Putin should by blood and force Revoke this sales and reclaim Sovereignty of Alaska from USA, just like he did Crimea.
http://en.wikipedia.org/wiki/Alaska_Purchase
Americans are afraid of war and death, weak pale coward and bankrupt. No balls nor strength to fight, a dummy paper tiger. No real friends and no strong allies. No match against Russia. Easy for Russia to take out eyeballs and break limbs of USA. Many enemies of USA will take opportunity to side with Russia. Arabs, Latin America, PRC, with feuds or serious conflicts of interest will surely do. Iran, North Korea, Cuba, Venezuela etc etc. Opportunistic India, Pakistan etc will switch side.
US friends are all bankrupt and ill, and also very coward. They will chicken out just like they did in Ukraine Crimea case.
Once Putin smoke up USA cities and had hundreds of millions of civilian casualties, US military will be unable to even cope with rescuing their own. Surely have to give up Alaska and surrender and remain under Putin's feet forever.
Alaska has plenty oil and gas let alone other precious resources, USA cheated the Russian Tsar in the cheap deal purchase, time for Putin to reclaim and annex Alaska sovereignty for national pride and interest. His men are brave and willing to war.