Even before NATO leaders gather in London to mark the alliance’s 70th anniversary Tuesday and Wednesday, Turkey has put a damper on celebrations.

In a sign of a growing divide between the alliance and its second-biggest military force, Turkey has balked at a defense plan for Poland and the Baltic states in order to register its anger over NATO’s refusal to meet Turkey's demands over its own defense. The move could stymie approval of the plans at the summit, to be attended by US President Donald Trump, Turkish President Recep Tayyip Erdogan and other heads of NATO states. 

The dispute comes at a time when the transatlantic alliance is already badly bruised. Trump has emerged as its sharpest critic, accusing other members of being “delinquent,” hinting the United States might not defend allies under attack and even raising the possibility of withdrawal from the pact. Earlier this month, French President Emmanuel Macron said last month the lack of clear US leadership means NATO is suffering from “brain death.” 

Turkey wants NATO to support its fight against the Syrian Kurdish People’s Protection Units (YPG), including designating the group a terrorist organization, before it signs off on plans to defend the alliance’s eastern flank against Russia. Turkey entered northeast Syria in October, forcing Trump to hastily evacuate troops who were battling the Islamic State (IS) alongside the Kurdish militia, without consulting European partners; this effectively turned over territory to Russia and the Syrian government.