Russia, China and the United States have all made it clear that they will not take nuclear weapons off the table and officials in all of these nations have made overt threats that force will be used to defend their interests.
Over the last several years we've witnessed some key developments that are reminiscent of the posturing that eventually lead to the start of the Great War in 1914.
Russian and Chinese submarines, for example, have been spotted within a few hundred miles of both, the eastern and western, U.S. shores. In 2012, residents of the west coast captured photos and video of what appeared to be a missile contrail just off Southern California at the very same time that President Obama was in meetings with Chinese leaders. Though the U.S. government later claimed that the contrail was an airplane, expert analysis suggests otherwise.