Washington(CNN) President Donald Trump's decision to knock London Mayor Sadiq Khan on Monday didn't come out of nowhere.
Trump and Khan have spent the last year sniping at one another, largely over Trump's proposed travel ban that would temporarily bar some Muslims from entering the United States. Trump has used the policy, stalled now in US courts, as an example of the sort of toughness that he says Khan refuses to show amid terrorism attacks that are riling the United Kingdom.
Khan has pushed back, calling Trump bigoted and saying that he doesn't need advice from the US President.
Here's how we got here:
Trump, in the aftermath of an attack in San Bernardino, California, calls in a statement for a "total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what the hell is going on."
Khan is elected as the first Muslim mayor of London.
Trump tells The New York Times that Khan could be allowed to enter the United States. "There will always be exceptions" to the ban, Trump says.
Khan tells BBC that he has no interest in Trump's offer: "I don't want to be the exception to be allowed to go to America."
Khan tells CNN's Christiane Amanpour that Trump's views of Islam "are ignorant."
A day after bombings in New York and New Jersey, Khan tells The London Standard that attacks are "part and parcel" of life in major cities.
"It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things," he said.
Trump's son, Donald Trump, Jr., resurfaces that September tweet after an attack at Westminster.
"You have to be kidding me?!: Terror attacks are part of living in big city, says London Mayor Sadiq Khan," he tweeted.
The next day, Khan tells Amanpour that he isn't going to reply to the President's son.
"I've been doing far more important things over the last 24 hours," he says.
Attacks at the London Bridge and Borough Market kill seven and injure almost 50.
Khan says that London residents should expect to see an intensified police presence in the wake of the attacks.
"There's no reason to be alarmed," he says.
Trump then takes Khan's remark out of context.
"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!'" Trump tweets.
A Khan spokesman replied: "He has more important things to do than respond to Donald Trump's ill-informed tweet that deliberately takes out of context his remarks urging Londoners not to be alarmed when they saw more police - including armed officers - on the streets."
Trump again hammers Khan, saying the mainstream media is abetting Khan's timidity.
"Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his "no reason to be alarmed" statement. MSM is working hard to sell it!"
Khan hits back later when asked by Britain's Channel 4 News if he thought Trump's state visit to the UK should be called off.
"I don't think we should roll out the red carpet to the President of the USA in the circumstances where his policies go against everything we stand for," Khan responded.
"When you have a special relationship it is no different from when you have got a close mate. You stand with them in times of adversity, but you call them out when they are wrong. And there are many things about which Donald Trump is wrong."