LONDON, Dec. 3 (AP): U.S. President Donald Trump and his NATO counterparts were gathering in London Tuesday to mark the alliance’s 70th birthday amid deep tensions as spats between leaders expose a lack of unity that risks undermining the military organization’s credibility. For the third summit in a row, Trump is expected to renew demands that European allies and Canada step up defense spending. Meanwhile, lamenting NATO’s “brain death” due to a lack of U.S. leadership, French President Emmanuel Macron says NATO needs “a wake-up call.” Macron insists that strategic questions must be addressed, like improving ties with Russia and how to handle an unpredictable ally like Turkey. In turn, Turkish President Recep Tayyip Erdogan has lashed out at Macron, and their very public argument bodes ill for a summit hosted by British Prime Minister Boris Johnson, who is deep into an electoral campaign and desperately wants to smooth things over. Ankara raised the ire of its allies by invading northern Syria, and for buying Russian air defense systems with powerful computers aboard that suck up data and would compromise the military equipment of allies if they were stationed nearby. Before heading to London, Erdogan suggested that Turkey might not back Poland and NATO’s Baltic allies — Estonia, Latvia and Lithuania — should they require defending unless the allies support Turkish concerns about Syrian Kurdish fighters, which Ankara sees as terrorists. That threat raises new questions about NATO’s commitment to its collective defense clause — Article 5 — under which all allies vow to come to the aid of a member under attack. The clause has only ever been activated once, after the 9/11 attacks in the United States. The two-day summit kicks off with receptions at Buckingham Palace and Downing Street later Tuesday. A short working session will be held at a golf resort in outer London on Wednesday. NATO Secretary-General Jens Stoltenberg and some leaders were taking part in a side event Tuesday morning Trump is due to hold separate talks with Macron, German Chancellor Angela Merkel, Danish Prime Minister Mette Frederiksen and Italian Premier Giuseppe Conte on the sidelines of the summit. Johnson is also set to host talks on Syria with Macron, Merkel and Erdogan later Tuesday.