US President Donald Trump and his Turkish counterpart, Recep Tayyip Erdogan, met for their first bilateral talks on Tuesday. In Erdogan's words, the meeting was to be "a new milestone" for relations between Turkey and the United States. The presidents were expected to discuss the latest developments in Syria, the extradition of Fethullah Gulen - Erdogan's main suspect in last July's failed coup attempt - and the fate of Reza Zarrab, an Iranian-Turkish businessman who is now on trial in the United States.

Why is Turkey uneasy about the US's arming Kurdish forces?

It is likely that the US's arming of Kurdish People's Protection Units (YPG) will likely be one of the most contentious issues on the agenda - especially if the Trump administration proves unwilling to revise its policy. The YPG is one of the most effective armed regiments of the Syrian Democratic Forces, who are now mobilizing their forces to liberate Raqqa - a stronghold of the so-called Islamic State (IS).

Turkey's government considers the YPG an offshoot of the outlawed Kurdistan Workers' Party (PKK), with which it has been locked in armed conflict since the 1980s. The United States has designated the PKK as a terror organization but not the YPG. Turkish officials fear that arms supplied to Kurdish fighters in Syria could end up being used by the PKK.

So how will they fight IS?

Starting last August, Turkey conducted its seven-month cross-border Euphrates Shield operation in northern Syria with local rebels under the Free Syrian Army banner, capturing several cities from IS. 

Turkey has since offered to conduct the Raqqa operation with the FSA.

What about Assad?

Though the presidents are not fans of their Syrian counterpart, Bashar al-Assad, Erdogan seems to have dropped his demand that his neighbor resign.

As Turkey has rekindled its relationship with the Kremlin following its downing of a Russian military jet in 2015, both countries are focusing on coordinating the end of Syria's civil war. Turkey, Russia and Iran have since launched a diplomatic process to bring representatives from both the Assad regime and the rebels together in Astana, the capital of Kazakhstan.

Turkish officials believe that no-fly zones in northern Syria could reduce displacement by limiting Assad's airstrikes on civilians. Earlier this month, delegates at the talks in Astana decided to create four de-escalation zones inside Syria. Trump and Erdogan will likely discuss the implementation of the peace process, as well as the de-escalation zones, Russia's role and Iran's involvement.

Who is Gulen?

Another issue that Erdogan is expected to bring up in his meeting with Trump is Fethullah Gulen, an Islamic cleric who has lived in self-exile in the US state of Pennsylvania since 1999.

A former ally of Erdogan's, Gulen now stands accused of forming a terrorist organization to infiltrate the Turkish state and topple his government in last July's unsuccessful coup attempt. Gulen denies these allegations.

Though the United States has acknowledged receiving a formal extradition request from Turkey's government, US officials say they need more hard evidence linking Gulen to the elaborate plans.

Erdogan is expected to ask Trump directly to extradite Gulen.

And who's Zarrab?

Reza Zarrab was arrested in the United States over a year ago and is being held in jail pending trial on charges of laundering money and violating the US's sanctions against Iran. Zarrab is well-connected in Ankara and widely known in Turkey, where he was arrested for 70 days after his name was embroiled into a vast graft and bribery operation in late 2013. He was later acquitted.

In late March, former New York Mayor Rudolph Giuliani - a strong and early Trump ally - and former US Attorney General Michael Mukasey joined Zarrab's defense team in the United States.

Court documents made public in April revealed that Giuliani and Mukasey had held a meeting with Erdogan regarding Zarrab's case in late February and attempted to initiate his release through a diplomatic process.

Judge Richard Berman has asked the defense team to clarify what roles Giuliani and Mukasey have taken on, and said during a hearing last week that the case should be resolved through legal means - not through diplomatic initiatives.