Canadian PM Justin Trudeau -- EPIC INTERNATIONAL SMACK TALK ... Calls Out Obama & Royals (VIDEO)

Canadian PM Justin Trudeau has plunged Canada into a world war of international trash talk ... with a video he made calling out the U.S. and Great Britain.

Those two countries have been embroiled in a NATO-threatening battle of words since last Friday ... all because of the Invictus games -- an international sporting competition between wounded servicemen.

Barack & Michelle Obama fired the first smack shot via social media, calling out Great Britain ... and dropping the mic.

Of course the Brits would never surrender or appease (lol) ... so Prince Harry struck back -- and brought the FREAKIN' QUEEN HERSELF into it ... who royally scoffed at Obama's shot.

We gotta say ... kudos to the Canucks for actually having your PM himself hit the deck for a couple of pushies ... which makes the mic drop a little more swag.