US pastor Andrew Brunson was first arrested in October 2016, when Turkey was in the immediate throes of a crackdown on suspected instigators of the failed July coup attempt. He is accused of supporting exiled cleric Fethullah Gulen as well as Kurdish militants. Ankara claims Gulen masterminded the coup bid.

Brunson was charged of "committing crimes on behalf of terror groups without being a member" and espionage. The 50-year-old pastor spent two years in jail, facing a prison sentence of up to 35 years if convicted on both counts.

In July, 2018, Brunson was allowed to leave prison and remain under house arrest, but his request to leave Turkey was denied by authorities. Washington has maintained that there is no credible evidence to support the charges brought against him. "Release Pastor Andrew Brunson now or be prepared to face the consequences," US Vice President Mike Pence warned Turkey.

US President Donald Trump threatened to slap "large sanctions" on Turkey if it refused to free the Christian pastor. Trump took to Twitter to demand that the pastor be freed, writing: "This innocent man of faith should be released immediately!" In a tit-for-tat tweet, Turkish Foreign Minister Melvut Cavusoglu responded by saying that, "No one dictates [to] Turkey."

In early August the White House announced it was imposing sanctions on two top members of the Turkish government – Justice Minister Abdulhamit Gul (above right) and Interior Minister Suleyman Soylu – for their roles in Brunson's detention. President Erdogan has said that he will not be swayed by sanctions.

But signs of a diplomatic thaw began to appear after US Secretary of State Mike Pompeo and Turkey's Cavusoglu met in Singapore on the sidelines of a regional summit on August 3. "They [Pompeo and Cavusoglu] spoke about a number of issues and had a constructive conversation," State Department spokeswoman Heather Nauert said.

Eventually, a court in Turkey's Aliaga found Brunson guilty on terror charges and sentenced him to over three years in prison. At the same time, however, the court said he would not serve the term due to the time he had already spent behind bars. The preacher was allowed to leave the country.

The Brunson row is the latest in a series of diplomatic and military disputes between the two NATO allies. The US uses bases in Turkey for its military operations across the Middle East, but the two countries have sparred over numerous issues, including Washington's support for Kurdish fighters in Syria, whom Ankara sees as a threat to its political stability.

May 16, 2017: Trump welcomes Erdogan to Washington, saying both presidents have a "great relationship" and would make it "even better." Erdogan congratulates Trump on his "legendary" 2016 election win but complains bitterly about US arming of the Kurdish YPG militia, claiming that its inclusion in the US-led campaign against IS in in war-torn Syria provides a cover for Kurdish separatism.

May 17: As Erdogan ends his visit, Voice of America video footage emerges showing his guards assaulting Kurdish protesters outside the Turkish ambassador's residence in Washington. A month later, US authorities issue arrest warrants for 12 members of Erdogan's security detail, who had long returned to Turkey. US Secretary of State Rex Tillerson says the assaults breached "legitimate" free speech.

July 15, 2017: Turkey marks the first anniversary of the failed coup attempt. In a post-coup bid crackdown 50,000 people were arrested, accused of links to the US-based cleric Fethullah Gulen, an Erdogan ally-turned-rival. Tens of thousands more face job suspensions. The refusal of the US to extradite Gulen has been a major sore spot in relations.

August 23: US Defense Secretary James Mattis visits Ankara as the Pentagon stresses US commitment to bilateral relations and "honest dialogue." Mattis had just visited Iraq to assess the anti-IS campaign. Erdogan tells Turkish media that Turkey will thwart any attempt by the Kurdish People's Protection Units (YPG) to establish a "terror corridor" in northern Syria through to the Mediterranean.

October 5: Turkish authorities arrest Metin Topuz, a Turkish national employed at the US consulate in Istanbul. He is formally charged with espionage and collaboration in the 2016 coup attempt. The US embassy in Ankara subsequently says it is "deeply disturbed" by the arrest. It's reportedly the second since March, when a Turkish US consulate employee was arrested in Adana.

October 8-9: The United States suspends its issuance of non-immigrant visa applications to Turkish nationals, saying it has to "reassess" Turkish readiness to respect security at US diplomatic missions. Turkey suspends its visa services for US nationals and summons another staffer at the US consulate in Istanbul.

November 6: The US Embassy in Ankara announces that it is reinstating its visa program for Turkish tourists on a "limited" basis after receiving assurances from the government that no employees will be detained "for carrying out official duties." Shortly thereafter, Turkey confirms that it is also resuming visa services for US citizens one day before Prime Minister Yildirim visits Washington.

December through August, 2018: In December, Turkey announced it would buy the Russian S-400 missile system, which is incompatable with NATO systems. The US Congress has included a provision in a defense bill that would cut Turkey out of the F-35 fighter jet program if it moves forward with the S-400 deal.

August 1, 2018: The US sanctions Turkey's interior and justice ministers over the continued detention of pastor Andrew Brunson. Brunson had been moved from prison to house arrest in late July, but that fell short of US demands for his immediate release and end to terror and espionage charges. Brunson was arrested almost two years ago.