Under the formula, countries would pay the full cost of stationing American troops on their territory, plus 50 percent more, said U.S. and foreign officials familiar with the idea, which could have allies contributing five times what they provide.

Trump calls the formula “cost plus 50,” and it has struck fear in the hearts of U.S. allies who view it as extortionate.

Rumors that the formula could become a global standard have especially rattled Germany, Japan and South Korea, which host thousands of forces, and U.S. officials have mentioned the demand to at least one country in a formal negotiation setting, said people familiar with the matter…

“Trump is correct in wanting U.S. allies to bear more responsibility for collective defense, but demanding protection money from them is the wrong way to do it,” said Stephen Walt, a scholar of international relations at Harvard University. “Our armed forces are not mercenaries, and we shouldn’t send U.S. troops into harm’s way just because another country is paying us.”