When the Union was fighting to preserve itself in the Civil War, the France of Napoleon III moved troops into Mexico, overthrew the regime of Benito Juarez, set up a monarchy and put Austrian Archduke Maximilian von Habsburg on the throne as Emperor of Mexico – one month before Gettysburg.

Preoccupied, the Union did nothing.

At war's end, in 1865, however, at the urging of Gens. Ulysses S. Grant and William Sherman, the Union sent 40,000 troops to the Mexican border.

Secretary of State William Seward dispatched Gen. John Schofield to Paris with the following instructions: "I want you to get your legs under Napoleon's mahogany and tell him he must get out of Mexico."

The U.S. troops on Mexico's border convinced Napoleon to comply, though Maximilian bravely refused to leave and was captured and put before a firing squad.

The point of the episode for today's crisis in Ukraine?

A powerful army on a nation's border can send a message and dictate terms without going in and without going to war.

Whether Russian President Vladimir Putin intends to send his 100,000 troops now on the Crimean, Donbass, and Belarusian borders of Ukraine into the country to occupy more territory we do not know.

But the message being sent by the Russian army is clear: Putin wants his own Monroe Doctrine. Putin wants Ukraine outside of NATO, and permanently.