Trump exposes the players. Always the business man keeping it professional but he gives the players a stage, they can either do the right thing or not. If he were to go head to head then it's a fight, but if he gives the players a stage then the audience/citizens become the "jury".
I think he was trying to rouse Canada's nationalism, to get citizens to unite. The enemy is great and we are on the same continent.
He also may be alluding to an issue which resulted in a Treaty in 1871 when referring to Canada becoming part of the U.S. Although Canada hadn't done anything, England had and the U.S. wanted repayment:
 
 >>/152446/ The Golden Rule pb

1871 TREATY BETWEEN THE UNITED STATES AND GREAT BRITAIN
The Alabama Claims, 1862–1872
The United States demanded compensation from Britain for the damage wrought by the British-built, Southern-operated commerce raiders, based upon the argument that the British Government, by aiding the creation of a Confederate Navy, had inadequately followed its neutrality laws. The damages discussed were enormous. Charles Sumner, Chairman of the Senate Foreign Relations Committee, argued that British aid to the Confederacy had prolonged the Civil War by 2 years, and indirectly cost the United States hundreds of millions, or even billions of dollars (the figure Sumner suggested was $2.125 billion). Some Americans adopted this argument and suggested that Britain should offer Canada to the United States in compensation. Such proposals were not taken seriously by British statesmen, but they convey the passion with which some Americans viewed the issue.

Trump's concern could actually be England and England's presence in North America (the continent). He could be warning Canada to saddle up and get on the horse, it's time to fight the good fight. 

Election fraud is world-wide, no one has voted for anyone in the past 25 years unless an election happened to be too big to rig. [qr bunker Election Fraud Network]

but what does a frog know.