>>Tsar Alexander II sold Alaska... He had no trouble believing that the expansion had gone far enough, and that Alaska was going to be a hard place to hold.

The Russian Empire had overextended into Alaska. Russian survival depended on expanding out to defensible geographic features (rivers, mountain chains, seas, oceans) as Russia was both flat and relatively poor farmland so it could not support an army large enough to fend off attacks on that broad, flat terrain. Alaska added nothing to their defense, just more territory not unlike parts of Siberia across the Aleutians.