A Microsoft Office logo sits on the touchscreen display of a Nokia Lumia 720 Windows smartphone.

After just three years of massive restructuring that was supposed to revive Nokia, the former global mobile communications giant, the company's gone. Why?

Recently, Microsoft agreed to acquire the handset and services business of Nokia for about $7.2 billion. In the process, Stephen Elop, the former Microsoft executive who ran Nokia until the deal was signed, will rejoin Microsoft, which some observers believe sets him up as a potential successor for its CEO Steven A. Ballmer.