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NATO was founded 70 years ago. But as Western leaders gather in London for a summit starting on Tuesday, the transatlantic alliance looks more vulnerable than venerable.

Some external factors include transnational threats such as terrorism that defy defined military solutions, as well as a revanchist Russia, whose military provocations amplify its asymmetric political attacks, including interfering in Western elections.

But the biggest threat may be internal. The U.S. is the pact's longtime leader, but President Donald Trump has been withering in his criticism. Trump, who once labeled NATO "obsolete," has sown uncertainty over the alliance's Article 5, the collective-defense mechanism that's only been triggered to defend the U.S. after 9/11 — indeed, NATO troops are still in Afghanistan as Trump signals a willingness to return to peace talks with the Taliban. Trump has also castigated allies over many not yet spending 2% of GDP on defense, although all 29 members have pledged to meet that goal by 2024.

Trump's domestic political problems are sure to shadow the summit, too. In fact, they were raised by the president himself on Saturday, when he tweeted his complaint that the impeachment inquiry will proceed while he's in London. But holding the hearings is actually a sign of an accountable democracy, standing in stark contrast to Soviet totalitarianism and the sham democracy of modern-day Moscow that NATO was founded to counter.

The summit's hosts are undergoing a healthy democratic exercise, too: a parliamentary election slated for next week. It comes amid the country's continued convulsion over 2016's Brexit referendum, in which British voters narrowly chose to leave the European Union, the political entity that undergirds NATO's military alliance.

Turkey is sure to be a sore subject as well. Its illiberal leader, President Recep Tayyip Erdogan, is feuding with multiple countries over multiple issues. His purchase of a Russian-made anti-aircraft missile system is antithetical to the alliance itself. Another issue splitting Ankara from Washington and other Western capitals is the Turkish government holding up a NATO defense plan for Poland and the Baltic countries until it receives more support for its fight against Syrian Kurdish YPG militia, which fought with allies against ISIS but which Turkey considers a terrorist group.

The spat over the YPG led Erdogan to say that French President Emmanuel Macron should "check whether you are brain dead."

Erdogan was no doubt referring to Macron's provocative comments in the Economist magazine, in which he claimed that NATO was experiencing a "brain death." Macron was brushed back by several European leaders, including the ever-steady chancellor of Germany, Angela Merkel.

While Macron may have overstated the state of NATO relations, his comments reflect just how frayed the alliance is. The national leaders gathering in London this week should work to mend the internal issues, and focus on the external ones, like Russia, that pose the true threat to Western and indeed global stability.