ISTANBUL (AP) — Turkey made a surprise pledge to drop its opposition to Sweden joining NATO, paving the way for the Nordic country to become a member of the Western military alliance.

NATO Secretary General Jens Stoltenberg heralded the agreement Monday after talks with Turkish President Recep Tayyip Erdogan and Swedish Prime Minister Ulf Kristersson.

Stoltenberg said Turkey had agreed to support Sweden’s NATO bid – by putting the issue to a vote in Parliament -- in return for deeper cooperation with Sweden on security issues and a pledge from Sweden to revive Turkey’s quest for EU membership. The agreement also says Sweden and Turkey will step up trade and investment with each other.

Hungary, the only other NATO holdout on Sweden, is also expected to drop its opposition. Hungary’s foreign minister said Tuesday that his country’s ratification of Sweden’s NATO membership was now just a “technical matter.”

Erdogan has been uncharacteristically quiet since the agreement was publicized, declining to comment on the reasons for his apparent change of heart on Sweden’s NATO membership.

It remains to be seen how quickly the issue will be taken up by Turkey’s Parliament.

Here are key factors, and possible incentives, that Turkey is considering as it weighs Sweden’s entry into NATO.

Erdogan's opposition to Swedish membership in NATO had focused heavily on his belief that the Nordic country has been too lenient toward elements of the Turkish and Kurdish diasporas that Turkey views as security threats — namely, people associated with militant Kurdish groups and others connected to a 2016 coup attempt against Erdogan.

An agreement signed between Turkey, Sweden and Finland last year set out to tackle some of these concerns, and Sweden recently tightened its anti-terrorism laws, making support for an extremist organization punishable by up to eight years’ imprisonment.

Sweden also says it has cracked down on the activities of people connected to the Kurdistan Workers’ Party, or PKK, which has waged a 39-year insurgency in Turkey.

Under the agreement outlined Monday, Sweden said it would also work against the Syrian wing of the PKK, known as the YPG.

A related issue for Erdogan has been anti-Turkey and anti-Islam protests held in Stockholm, some of which involved the burning of the Quran.

Although condemned by the Swedish government, the demonstrations drew strong reaction from Turkey, which criticized Sweden — which has strong free-speech protections — for “allowing” public displays of anti-Muslim sentiment.

But how Sweden deals with Kurdish militant groups has always been way more important for Turkey than the protests, said Cigdem Ustun, secretary general of the Center for Economics and Foreign Policy Studies in Istanbul.

Ustum described Turkey’s statements over the Quran-burnings as “a tool in the negotiations.” adding that “for Turkey it was mainly about terrorist organizations.”

Erdogan's promise to Stoltenberg to bring Sweden’s NATO membership for a ratification vote in the Turkish Parliament may also have been linked to Turkey's efforts to upgrade the country's fleet of F-16 fighter jets.

Turkey has asked the U.S. for approval to buy 40 new F-16s, as well as kits to upgrade its existing fleet. The request was backed by the White House but ran into opposition in Congress. Both U.S. and Turkish officials have insisted that any such deal would not be linked to Sweden’s NATO membership.

But within hours of Erdogan apparently dropping his veto on Sweden, President Joe Biden signaled that the U.S. intended to proceed with the F-16 sale in consultation with Congress. In a statement welcoming Erdogan's agreement to bring Sweden's NATO bid before Parliament, Biden said he will work with Turkey “on enhancing defense and deterrence in the Euro-Atlantic area."

The F-16 deal’s biggest critic, Senate Foreign Relations Committee Chairman Bob Menendez, D-N.J, appeared to soften his stance, saying “there may be a way forward.” Menendez has opposed any F-16 deal over the potential threat to Turkey's neighbor Greece.

The F-16s “have been an important part of the negotiations although there’s no formal link," said Ustun. "It’s obvious there’s been some progress in that regard.”

Early on Monday, Erdogan made a surprise warning that he would block Sweden’s attempt to become a NATO ally unless European members of the military organization “pave the way” for Turkey to join the world’s biggest trading bloc.

It was the first time that Erdogan had linked the two countries’ aspirations in this way.

Following his remarks, European officials rebuffed any link between Sweden’s NATO membership and Turkey's 36-year bid to join the EU.

“You cannot link the two processes,” European Commission spokesperson Dana Spinant said.

Stoltenberg reiterated the point, saying that Turkey’s relationship with the EU was “not an issue for NATO, it’s an issue for the European Union.” But, he told reporters that “what Sweden agreed today as an EU member was to support actively the efforts to reinvigorate Turkey’s EU accession process.”

Sweden also said it would seek improved customs arrangements and take steps to implement visa-free European travel for Turkish citizens.

Still, few expect any accelerated movement on Turkey's EU membership talks, which stalled in 2018 because of the country's democratic backsliding and poor record on human rights.