ANKARA, TURKEY • President Joe Biden and Turkish counterpart Recep Tayyip Erdogan have known each other for years, but their meeting Monday will be their first as heads of state. And it comes at a particularly tense moment for relations between their two countries.

The list of disagreements is unusually long for the two NATO allies: There’s U.S. support for Kurdish fighters in Syria, as well as Turkey’s purchase of a Russian weapons system. And in April, Biden infuriated Ankara by declaring that the Ottoman-era mass killing and deportations of Armenians was “genocide.”

Previous U.S. presidents had avoided using the term out of concern that it would complicate ties with Turkey, which is fiercely proud of its Ottoman history and insists that those killed in the early 20th century were victims of civil war and unrest.

However, besides blasting the decision in speeches, Erdogan didn’t immediately hit back at Washington.

The muted response suggests he wants a good relationship with Biden, said Rachel Ellehuus, an analyst at the Washington think tank Center for Strategic and International Studies.

“Not least because he needs that economic relationship with the U.S. and the appearance of a cooperative relationship in order to retain his base, which is very much built on a functioning Turkish economy that is tethered into the West,” Ellehuus said.

However, before leaving Sunday for the NATO summit in Brussels where he will meet Biden, Erdogan described the president’s comments on the killings of Armenians living in the Ottoman Empire as “very negative” and an “approach (that) has seriously upset us.”

Erdogan, in power for 18 years as prime minister and then president, also one-on-one meetings with French President Emmanuel Macron, German Chancellor Angela Merkel, Greek Prime Minister Kyriakos Mitsotakis and British Prime Minister Boris Johnson lined up during Monday’s summit. He told reporters he plans to “underline the importance we attach to the alliance with our allies.”