Bangladesh, May 15 -- As NATO prepares for its upcoming summit in Ankara this July, the alliance faces one of the most consequential moments in its modern history. The central issue dominating the agenda is no longer simply collective defense against Russia or continued support for Ukraine. Instead, it is a question that has haunted the alliance for decades: who pays for Europes security?

The issue of defense spending and burden sharing has become the defining test of NATO unity under the second administration of Donald Trump. While previous American presidents quietly expressed frustration over Europes low military spending, Trump has transformed the debate into a political and strategic confrontation that now shapes the future of the a...