How?
http://www.nbcnews.com/news/world/five-reasons-why-nato-nations-don-t-always-pay-their-n619966
“Jolyon Howorth, a visiting professor of political science at Yale University, says that it’s hard to see a scenario where other NATO members would increase their defense spending if they feel that the U.S. will continue to help carry the burden for everyone else.
“Unless and until the U.S. makes it absolutely clear to its EU allies that there is a limit to Washington’s largess, there will be little change,” Howorth said.
Simonyi, who is currently the managing director of the Center for Transatlantic Relations at Johns Hopkins University, largely agrees with this and argues that the U.S. should be more explicit in its expectations for member countries beyond reaching that 2 percent target.”