What did Ron Weasley's dad know about the Imperius Curse?

Question

When the fake Mad-Eye Moody taught a class on Defense Against the Dark Arts, he introduced the students to the Three Unforgiveable Curses. He calls out students and tells them to name one of the three curses.

Mad-Eye: Weasley!

Weasley: Yes?

Mad-Eye: Stand! Give us a curse.

Weasley: Well, my dad did tell me about one. Imperious Curse.

Mad-Eye: Oh, yeah. Your father would know all about that. Gave the ministry quite a bit of grief a few years ago. Perhaps this will show you why.

Why does the fake Mad-Eye Moody say Ron's father, Arthur Weasley, would know all about the Imperius Curse?

What happened with him a few years prior?

Please provide answers supported by quotes from canon sources.

Answer 1

The Ministry had to sort out which people who worked with Death Eaters were Imperiused.

Arthur Weasley, as an employee of the Ministry of Magic, would have been working there when the Ministry was helping determine after the first war which people engaged in Death Eater activities were under the Imperius Curse and which were doing it willingly.

“Years back, there were a lot of witches and wizards being controlled by the Imperius Curse,’ said Moody, and Harry knew he was talking about the days in which Voldemort had been all-powerful. ‘Some job for the Ministry, trying to sort out who was being forced to act, and who was acting of their own free will.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)}}

Many Death Eaters claimed to be Imperiused to avoid sentencing, and the Ministry had to figure out which people really were.

“Avery – from what I’ve heard he wormed his way out of trouble by saying he’d been acting under the Imperius Curse – he’s still at large.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

Everyone working at the Ministry would have likely been familiar with the Imperius Curse, because either they or their colleagues would have been involved in figuring out who was under it. Arthur might not have been actively participating in figuring out who were Death Eaters and who weren't, but he was still working at the Ministry where the effects of the Imperius Curse would have been common knowledge.

The Death Eaters did indeed use the Imperius Curse as part of their tactics, for example on Broderick Bode.

“Bode could never have taken it, Master … Bode would have known he could not … undoubtedly, that is why he fought so hard against Malfoy’s Imperius Curse …”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 26 (Seen and Unforeseen)}}

In addition, the Death Eaters also used it on Ministry officials like Pius Thicknesse, in order to gain power and control over the Ministry. So it seems plausible that, during the first war, all Ministry officials would be instructed to watch for signs of their colleagues being Imperiused.

“My Lord, I have good news on that score. I have – with difficulty, and after great effort – succeeded in placing an Imperius Curse upon Pius Thicknesse.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 1 (The Dark Lord Ascending)}}

They planned to get more Ministry officials under their control as well, so it seems likely that during the first war, the Ministry might have instructed its staff to watch for their colleagues possibly being Imperiused. If this was the case, then all departments would likely be given the same warnings, which would reach Arthur Weasley as well since he was a Ministry employee.

“It is a start,’ said Voldemort. ‘But Thicknesse is only one man. Scrimgeour must be surrounded by our people before I act. One failed attempt on the Minister’s life will set me back a long way.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 1 (The Dark Lord Ascending)}}

During the second war, for example, they had placed people within the Department of Magical Transport, possibly by having Imperiused them.

“We now have several people planted within the Department of Magical Transport. If Potter Apparates or uses the Floo Network, we shall know immediately.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 1 (The Dark Lord Ascending)}}

By using the Imperius Curse the Death Eaters are able to stage a successful and stealthy coup of the Ministry.

“The coup has been smooth and virtually silent,’ said Lupin. ‘The official version of Scrimgeour’s murder is that he resigned; he has been replaced by Pius Thicknesse, who is under the Imperius Curse.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)}}

Answer 2

The quote as you had it at first, was not quite correct. There's no "he", so it wasn't Arthur Weasley himself who gave the Ministry "a lot of trouble".

Here's the relevant quote from the book:

"So . . . do any of you know which curses are most heavily punished by wizarding law?"
Several hands rose tentatively into the air, including Ron's and Hermione's. Moody pointed at Ron, though his magical eye was still fixed on Lavender.
"Er," said Ron tentatively, "my dad told me about one. . . . Is it called the Imperius Curse, or something?"
"Ah, yes," said Moody appreciatively. "Your father would know that one. Gave the Ministry a lot of trouble at one time, the Imperius Curse."

_{Harry Potter and the Goblet of Fire, chapter 14, "The Unforgivable Curses"}

The implied missing pronoun is not "he", but "it", referring to the Imperius Curse itself. It was the Imperius Curse that gave the Ministry of Magic a lot of trouble, as understood by Harry a couple of paragraphs down in the same chapter.

"Years back, there were a lot of witches and wizards being controlled by the Imperius Curse," said Moody, and Harry knew he was talking about the days in which Voldemort had been all-powerful. "Some job for the Ministry, trying to sort out who was being forced to act, and who was acting of their own free will."

_{Harry Potter and the Goblet of Fire, chapter 14, "The Unforgivable Curses"}

Since Arthur Weasley works at the Ministry, he would indeed know all about that curse.

Answer 3

Everyone in the Ministry would've had some experience with fighting the Imperius Curse.

As the other answers here say, the Imperius Curse was being widely used by the Death Eaters during the First Wizarding War. I thought I'd add some additional information about why Arthur Weasley might've been expected to know about this specific Dark curse.

The picture that Sirius paints of the Ministry at that time is of utter disarray.

He walked once up the cave, back again, and then said, "Imagine that Voldemort's powerful now. You don't know who his supporters are, you don't know who's working for him and who isn't; you know he can control people so that they do terrible things without being able to stop themselves. You're scared for yourself, and your family, and your friends. Every week, news comes of more deaths, more disappearances, more torturing...the Ministry of Magic's in disarray, they don't know what to do, they're trying to keep everything hidden from the Muggles, but meanwhile, Muggles are dying too. Terror everywhere...panic...confusion...that's how it used to be."
_{(Goblet of Fire, Chapter 27, Padfoot Returns).}

We can learn a few things from this quote. Firstly, the Death Eaters were using the Imperius Curse on an almost routine basis. Nobody knew who was under it and everyone was under constant suspicion.

Secondly, the Ministry was under a lot of pressure during this time period. Much of their energy was spent in trying to keep an extremely violent and bloody wizarding war hidden from the Muggles. They succeeded in this but it would've taken an extraordinary amount of manpower. Those not hiding the war would've been involved in fighting it. It seems highly likely in this climate that Ministry employees would've been pulled off their day-to-day jobs to focus on the war effort. That means that pretty much all Ministry employees would've been forced to become familiar with the Imperius Curse, its symptoms and how to fight it.

Molly gives us a similar picture of a wartime Ministry which is under strain.

Neither Mr Weasley nor Percy was at home much over the following week. Both left the house each morning before the rest of the family got up, and returned well after dinner every night.
[...]
"Your father hasn't had to go into the office at weekends since the days of You-Know-Who," she said. "They're working him far too hard. His dinner's going to be ruined if he doesn't come home soon."
_{(Goblet of Fire, Chapter 10, Mayhem at the Ministry).}

Note the title of the chapter, showing that the Ministry was once again in a state of mayhem, as it was during the First Wizarding War. Note also the fact that Mr Weasley is overworked and continuously busy. The Ministry is fighting fires all over the place (literally, in Percy's case). He wasn't just doing the routine tasks of his ordinary work. He was also filling in for other colleagues and helping out as things came up. During the First Wizarding War this would've included fighting the Imperius Curse.

RichS asks in a comment why the Aurors wouldn't have done this work. The answer is that of course they did. But their focus was surely on frontline conflict, in fighting and arresting known Death Eaters. They wouldn't have had much time to spare to investigate suspected cases of the Imperius Curse.