President Donald Trump, following the lead set by his legal adviser Rudy Giuliani on Monday, is drawing a tricky line on the Russia investigation.

Month after month, Mr Trump’s line has been that there was no collusion between him (or his campaign or between some generally unidentified group) and Russian interference efforts on his behalf in the 2016 election.

On Tuesday, though, that evolved into this tweet by the president: “Collusion is not a crime, but that doesn’t matter because there was No Collusion (except by Crooked Hillary and the Democrats)!”

He has said similar things before, though rarely as explicitly. Setting aside the circuitous and hollow comment about Hillary Clinton and the Democrats, though, it’s important to reiterate a central point.

Collusion is not a crime, but colluding can be. In the colloquial sense, collusion simply means an act of secret cooperation, generally for nefarious ends. There’s no law you can violate that constitutes collusion in the third degree – but working with someone in a conspiracy to break the law is illegal.

It’s called conspiracy.

“Every conspiracy is a form of collusion,” Stanford law professor Robert Weisberg said in a telephone interview on Tuesday. “Conspiracy is an agreement to do a crime plus some overt act in that direction. It’s really that simple.” An advantage of the law from a prosecutorial standpoint, he added, was that “you can punish the agreement and the overt act even if no one comes very, very close to committing the crime.”

In other words, no crime even needs to have been committed. If you and I agree to rob Fifth Third Bank and then buy masks to conceal our identities – the overt act – we’ve engaged in a criminal conspiracy, even if we never rob the bank. “Overt acts” can be any number of things. Mr Weisberg said that conspiracy indictments often “list about nine million overt acts,” like making a phone call or looking something up online. “The overt act is a nothing requirement,” Mr Weisberg said.

The web of conspiracy can be dangerous for criminal actors in unexpected ways. According to the Pinkerton doctrine, if you steal a car to use as a getaway vehicle for our bank robbery without my knowledge, I, as your co-conspirator, can be found guilty of auto theft even if I didn’t know a car had been stolen.

“If you have a lot of actors in a conspiracy with lots of them doing ancillary acts which might help towards the accomplishment of the target act, lots of people in lots of places can be guilty of lots of crimes,” he said.

Colluding can also be illegal in another way.

“There’s accomplice liability law,” Mr Weisberg said. “Even if we didn’t have a conspiracy law, if you are engaged in an act with a certain mental state which would make you guilty of a crime and if I aid and abet you – and sometimes aiding and abetting can be really a form of really active encouragement, no more than that – I am an accomplice to the crime, and an accomplice to a crime commits the crime.”

“There isn’t any separate crime called ‘complicity’,” he continued. “It’s just a way of committing a crime.”

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A conspiracy must be in place before the crime at its heart occurs (if it does). The indictment obtained by special counsel Robert Mueller‘s team against alleged Russian hackers provided a timeline of that effort, beginning in early 2016. But it was on-going over the course of the year, with access to the Democratic National Committee continuing into October of that year. An effort to aid efforts to access opponents’ communications illegally could be both collusion and conspiracy.

But we can move closer to home for Mr Trump, speaking literally. It is illegal for a federal campaign to accept something of value from a foreign agent – and, according to Lawrence Noble, former general counsel for the Federal Election Commission, so is soliciting such a contribution. That raises questions about the meeting between a Kremlin-linked lawyer and Donald Trump Jr and other campaign insiders in Trump Tower in June 2016 – a meeting set up to provide “very high level and sensitive information [as] part of Russia and its government’s support for Mr Trump,” according to the first email Mr Trump Jr. received.

“If [members of the Trump campaign] were soliciting something of value, it may well have been illegal,” Mr Noble said of that meeting. “If they were told, ‘You go to this meeting and Russia will give you $2,700 for the campaign,’ and you went to that? It would be illegal – even if you didn’t get it.” Money has a direct monetary value, of course, but other things have monetary value, too. If the Trump team were offered information that had a cost to the Russian government and if the team solicited that contribution, that information knowingly and willfully, that could constitute a federal crime.

This is why the president’s awareness of the Trump Tower meeting could be important. If Mr Trump knew about the meeting, as seems likely, or if Donald Trump Jr actually got clearance from Mr Trump by phone before signing off on the meeting, which seems possible, the prospect of a criminal conspiracy looms. The crime would be that solicitation of valuable information; the overt acts, myriad.

We’re further down the road now than we need to be.

“The collusion term is a red herring,” Mr Weisberg said. “Both accomplice liability – which is the same thing as complicity – and conspiracy could easily be described as forms of collusion, and they’re both ways of committing crimes.”

Mr Trump’s argument is, to some extent, like saying that wiring money is not illegal. It’s not!

But wiring money to pay a hit man is.