Secretary of Defense James Mattis spent several days in meetings with his fellow defense ministers at NATO’s expensive new headquarters in Brussels this week. Mattis’ trip to Western Europe, however, wasn’t a care-free rendezvous. The retired four-star field and combatant commander carried a message from President Trump that all of the alliance’s members have gotten used to hearing: Europeans must be as concerned about Russia as Washington, and each and every single one of them should pay their fair share to contribute to the common defense of Europe.

It should be noted that extracting more defense dollars from European governments is not a Trump-specific initiative. Presidents as far back as George W. Bush have attempted to hammer home the point that burden-sharing is not just a theoretical concept; it’s indispensable to NATO as an organization and its credibility. Successive NATO Secretaries General have largely paid lip service to the burden sharing argument, mainly because U.S. leaders have cold, hard math on their side.