Since the time of Genghis Khan. Since the time of Alexander.
A good comparison, what with their massive stockpiles of cruise missiles, nuclear warheads and array of spy satellites.....
Most countries have given up on the whole empire thing with the US being the exception. Former imperial powers have learned the hard way and aren't in any rush to repeat the mistakes of the past so it's fair to assume that America's empire will be the last and therefore one of the biggest obstacles to peace. There may still be civil wars and border wars between neighbors but nothing that is comparable to the destablization an empire can create.
America's power is unprecedented. It occupies more countries than any other nation before in history (combat troops in 120+ countries), has declared the entire world it's battlefield and has sold more weapons to other countries than any other nation has in history. It's quite obviously the primary instigator of global conflict.