President Trump’s imposition of long-awaited sanctions on Turkey this week over its purchase of a Russian S-400 missile defense system illustrates the fragile state of a critical relationship — but it may also allow President-elect Joe Biden to start fresh with Turkish President Recep Tayyip Erdoğan.

Why it matters: Erdoğan raged against the sanctions, which target Turkey’s defense procurement agency and the agency’s leaders, as a “hostile attack” from a NATO ally. Trump had resisted pressure to impose them, but Congress forced his hand.

The big picture: While Erdoğan maintained a strong personal relationship with Trump over the last four years, many others in Washington were growing increasingly concerned by his muscular regional policies and warming ties with Russia.

• Enter Biden. His promises to revitalize multilateralism and prioritize relations with countries that “share our democratic values” — not to mention his skewering of Trump for embracing “all the thugs in the world” — pose very different challenges for Erdoğan.
• The two also have a history. As vice president in 2014, Biden called Erdoğan an "autocrat" and said he would support the opposition's efforts to defeat him. (Biden later issued an official apology).

What to watch: Soner Cagaptay, a fellow at the Washington Institute and author of "Erdoğan’s Empire," expects Erdoğan to open with a charm offensive.

• The Turkish president is a “shapeshifter” who “becomes what every U.S. president wants to see in their Turkish counterpart” — in Biden’s case, an “internationalist, reformer, healer.”
• “He wants to reverse this narrative of free-falling ties,” Cagaptay says, because it’s damaging for Turkey’s struggling economy and leads others to question whether the U.S. military really stands behind Turkey.

Biden has incentives of his own to patch things up. Turkey could play the role of either facilitator or spoiler in two of Biden’s top foreign policy focuses: Iran and Russia.

• And while Biden will include human rights and democracy in the relationship, Cagaptay says, Erdoğan will attempt to placate him, perhaps by releasing some political prisoners.

Yes, but: This will be a deeply complicated relationship, and it could quickly turn contentious.

• Russia, which has massive geopolitical and economic leverage over Turkey, wants to expand the U.S.-Turkey divide. Moscow could pressure Ankara to switch the S-400 system on. That would provoke both the U.S. and NATO, which views the Russian system as a threat.
• Erdoğan would likely choose a fight with Biden over one with Putin, Cagaptay says. He’s already involved in squabbles with the EU over immigration and gas exploration that could also pull in the Biden administration.
• Further crackdowns by Erdoğan on the political opposition would also generate backlash.

The bottom line: “Erdoğan’s relations with U.S. presidents start well,” Cagaptay says. “They never end well.”