There is a boiling feud between US President Donald Trump and Mayor of London, Sadiq Khan.

The most recent you’ll hear or read of it is the Mayor calling on the British government to cancel plans to receive Trump in London in a forthcoming state visit. He stated his sentiments in an interview with UK’s Channel 4 News days after the recent terror attack on London when he said, “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. 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. There are many things about which Donald Trump is wrong“.

That the British government will take Khan’s advice seriously and withdraw Trump’s invitation to the UK is not a given but what is certain is that Trump has further proved himself as an unreliable ally of Europe. For those who are confused about why Trump is “suddenly” propelled to unleash fury on the London Mayor, you’ll need to understand that the war of words between these two leaders dates back to a time before they even became “Mayor” or “President”.

Here’s the backstory:

On Saturday, 3 June, three knife-wielding men drove into pedestrians on London Bridge then launched an attack on revellers in the busy Borough market leaving 7 dead and over 40 badly wounded. This was the third terror attack in the UK in three months. In his response to this tragedy that befell his city, the Mayor of London, Sadiq Khan told Londoners to brace up and not “cower in fear”. He then said, “Londoners will see an increased police presence today and over the course of the next few days. There’s no reason to be alarmed“.

It was a fairly good speech in such dark moments but US President Donald Trump was beyond pissed at Khan for his words that he immediately took to Twitter.

At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!"

— Donald J. Trump (@realDonaldTrump) June 4, 2017

Media coverage has made it clear alongside Khan’s media team that Trump took the phrase: “no reason to be alarmed” out of context. Everyone else got the message, really. Khan only meant that Londoners should not be alarmed by the heavy police presence in London, and he knew not to spare any energy for trading words with Trump at a time when his city needed him the most so his spokesperson made this clear: “[The mayor] has more important things to do than respond to Donald Trump’s ill-informed tweet that deliberately takes out of context his remarks“.

Trump’s insensitivity and aggressiveness continued when he shared another riposte:

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!

— Donald J. Trump (@realDonaldTrump) June 5, 2017

This was on Monday and it seemed Khan had had enough so he took up Trump by himself so he said in a BBC interview, “Some people thrive on feud and division. We are not going to let Donald Trump divide our communities...”.

It was not the first time Khan had publicly alluded to Trump’s inherent ability to be divisive. In 2015, he said about Trump, “His views are divisive, they are outrageous, and I hope he loses. Badly.”. The latter had not even won the Republican primaries at this time but he had been extremely vocal about a blanket ban on Muslims. It was after a mass shooting in San Bernardino, California perpetrated by a Muslim extremist couple that left 14 dead and 22 seriously injured. Trump, of course, asked for “a complete shutdown on Muslims entering the US“.

In that same period, Khan was running for Mayor in London and as a Member of Parliament and former attorney who had fought for minority groups and was a strong voice for British Muslims, so it was obligatory for Khan to hit back at this ignorant US presidential candidate who had insulted everything he ever fought for.

Khan won the Mayoral election and when Trump was asked if he would allow Khan into the US if his proposed ban on Muslims succeeded, Trump said, “There will always be exceptions“.

But Trump apparently missed it again, he had clearly undermined Khan’s advocacy. It wasn’t about Khan alone so singling one man out was out of the point. Aside calling Trump “ignorant”, he said, “This isn’t just about me. It’s about my friends, my family, and everyone who comes from a background similar to mine, anywhere in the world.”

It look less than 24 hours for Trump to find out the Mayor had insulted him. In an interview with Piers Morgan on Good Morning Britain, Trump challenged Khan to an IQ test after hearing the man had called him ignorant and he said “I will remember those statements“.

Not only has Trump kept his word to never forget Khan’s statements, he passed on the vitriol to his son, Donald Trump Jr.

London experienced another terror attack on Westminster Bridge that claimed 4 lives and wounded over 40 in March 2017. The younger Donald took the opportunity to fish out a 6-month old interview where the Mayor had said larger cities were more prone to attacks and urged Brits to be more alert.

Trump Jr. tweeted: “You have to be kidding me?!: Terror attacks are part of living in big city, says London Mayor Sadiq Khan“.

From all indications, Trump is not set to ease up on blaming all Muslims for extremists’ activities and Khan, on the other hand will continue to use his office to amplify his advocacy against extremism and terrorism.