The highly anticipated meeting between Trump and Erdogan kicked off at the White House on Wednesday, where the stakes are high for an agreement between the two NATO allies.

Erdogan arrived in Washington on Tuesday, and is staying at the Willard Hotel – just one block from the White House. Since his arrival there’s been a massive security presence surrounding the building and blocking off the area.

The increased security is likely a result from his last visit to DC in 2017. That’s when clashes broke out and Erdogan’s body guards violently assaulted both Secret Service agents, and a group of protesters that included women and children.

So far, we have yet to see any violence today, but tensions are high.

Ahead of this visit, a senior administration official for the US said, “The President is committed to direct engagement and diplomacy where it is most critical: Erdogan himself. He believes in working the hardest on the hardest issues. Full and frank engagement is essential to addressing the challenges facing this relationship.”

One of those challenges is the ongoing conflict in Syria. Turkey launched a military offensive in northern Syria last month, against the Kurds, but it agreed to pull back following talks with Russia.

The United States initially pulled back troops from Syria’s border, and turned over control of thousands of ISIS prisoners in the region. But even though Trump has repeatedly said he does not want the US to police the region, he has also threatened Turkey with sanctions over its treatment of the Kurds, and he has warned against the release of those ISIS prisoners.

But Erdogan has said he is not happy with the deal that was made, and he accused Russia and the US of failing to follow through on their promises in the region. Erdogan has also threatened to send more than 1,000 ISIS prisoners back to their home countries in the West, even if their citizenship has been revoked.

While tensions remain in Syria, the stakes are also high for the future of weapons deals between the US and Turkey.

The US has been very vocal in its criticism of Turkey’s decision to purchase Russia’s S-400 air defense system earlier this year. In addition to removing Turkey from the F-35 program, US officials have also threatened Turkey with sanctions if it moves forward with using the S-400.

But a senior administration official said there may still be hope for Turkey to return to Washington’s good side—but only if it cuts ties with Russia.

He said, “There’s tremendous upside in this bilateral relationship in economic terms, a key part of which is the F-35 and Turkey’s role and potential role in the F-35 program. But to get there, we, as allies, need to resolve this issue of the S-400.”

Erdogan has already said he is planning to have a phone call with Russian President Putin about his frustrations with the situation in Syria, after he meets with Trump. But today’s visit could also force Erdogan to decide once and for all, between keeping the Russian equipment, or re-aligning with his NATO ally at the risk of sanctions.