Of all Trump’s policy unorthodoxies, demanding that Europe boost defense spending is the most justifiable. Among 28 members, just five — five — complied with the NATO guideline last year of devoting two percent of GDP to defense.

Now here’s Mattis earlier today in Brussels with a message that came straight from the boss:

“I owe it to you all to give you clarity on the political reality in the United States and to state the fair demand from my country’s people in concrete terms,” Mattis said. “America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to the alliance, each of your capitals needs to show its support for our common defense.”…

Mattis, a retired Marine general, recalled Wednesday that when he was NATO’s supreme allied commander of transformation from November 2007 to September 2009, he watched as then-Defense Secretary Robert Gates warned NATO nations that Congress and the American people “would lose their patience for carrying a disproportionate burden” of the defense of allies.

That impatience, Mattis said, is now a “governmental reality.”

“No longer can the American taxpayer carry a disproportionate share of the defense of Western values,” Mattis said. “Americans cannot care more for your children’s security than you do. Disregard for military readiness demonstrates a lack of respect for ourselves, for the alliance and for the freedoms we inherited, which are now clearly threatened.”

The criticism of that position is predictable and understandable, but so is the response:

The timing of this could hardly be worse. The US threatens to retreat from most important alliance that checks Russian aggression? Today? https://t.co/28N0ym8OWD

— Joe Scarborough (@JoeNBC) February 15, 2017

Okay, but if not now, when? What better time could there be to concentrate Europe’s minds on defense spending than with Putin looking west to Ukraine and beyond, installing intermediate-range missiles aimed at Europe inside Russia in violation of a treaty, and engaging in petty provocations against the U.S. to test Trump’s resolve? If you want to squeeze NATO members to pony up, choosing a moment to do so when they’re more worried about Russia than they’ve been in years seems opportune.

On the other hand, there’s a reason (well, two reasons) that the U.S. has traditionally let Europe slide in missing its target on NATO spending. One, of course, is the fear of re-militarization. WaPo noted in passing in its story about Mattis’s speech that if an economic powerhouse like Germany were to boost defense spending to two percent of GDP, its military would suddenly be better funded than Great Britain’s, historically not a harbinger of lasting peace and perhaps especially dangerous with nationalist movements gaining traction across the continent right now. In theory a militarily muscular Europe is bad for Russia; in practice, European nationalists tend to be Putin sympathizers and may encounter more antagonism among themselves than with Russia. Reducing the risk of war in eastern Europe with Moscow needs to be weighed against increasing the risk of war long-term between European countries.

The other reason the U.S. has traditionally let Europe slide is that there’s never really a good time to have this standoff. Trump has now made an ultimatum, with which NATO members will hopefully comply. If they don’t, though, and end up calling Trump’s bluff, the U.S. will be in a jam in which Trump will either have to lose face by maintaining America’s NATO commitment despite the defiance on spending from fellow members or he’ll have to show he means business by ratcheting down America’s contributions to NATO, which is sure to tempt Russia westward. Meanwhile, some members are destined to complain that they “can’t” meet the two-percent guideline, or at least not yet; their money’s tied up in welfare-state obligations at the moment, and while they’re happy to undertake the process of re-budgeting, that’ll take time. Is Trump willing to accept a promise to meet the guideline down the road as fulfillment of each member’s obligation or do they need to pony up ASAP? Can they “pay” their obligation to NATO in the form of indebtedness to the United States? It’s fine to shrug here and say “if they can’t or won’t pay, let ’em enjoy Russian occupation,” but Russian expansionism westward comes with costs for the United States too. That’s … why NATO exists in the first place.

Here’s Mattis today with NATO leader Jens Stoltenberg emphasizing again that member nations need to pay up. Russia seems increasingly sour towards Trump, especially now that Mike Flynn is gone, but the prospect of a rupture in NATO over spending should brighten their day a bit.