There was a good reason why the Roman Senate forbade the army from entering the city. Armies have a single purpose: to dominate and control. That’s what armies do: They march forward and vanquish enemies. Or there’s not really much point in having one.

From the grand events of antiquity to the humdrum affairs of Labour’s internal politics.

Momentum, Jeremy Corbyn’s Praetorian Guard, was created out of the remarkable insurgency that propelled him to the Labour leadership back in 2015.

It made sense for the Corbnyintes to try and bottle that enthusiasm and organisation, but Momentum was, from the very beginning, created as a standing army outside of the party’s control.

A back-up plan. If Corbyn was usurped by his internal opponents, Momentum could rely on hundreds of thousands of members and graduate into a new left-wing political party.

But June’s general election result has made Corbyn unassailable. His critics have withered. There is no realistic threat to his position, which begs the question: What is Momentum now for? Does it find itself without a purpose, or is it preparing the cross the Rubicon and seize control of Labour’s internal workings?

There have been skirmishes over the past few months, with local branches and constituencies across the country falling under the hard left’s influence. Meanwhile, Momentum’s founder, Jon Lansman, is currently running for a seat on the party’s National Executive Committee.

And while it’s likely that a swathe of moderate councillors will be replaced by Momentum supporters next year, robust local government regulations will prevent the hard left from being able to force through illegal budgets and the like.

But Momentum has bigger ambitions and the mandatory reselection of MPs remains the Holy Grail.

So far, Jeremy Corbyn has been incredibly cautious about triggering a full-on civil war with his MPs over this, but if Theresa May presses ahead with the parliamentary boundary changes for the next election, Labour MPs will, de facto, face mandatory reselection.

Indeed, if she wants to bequeath a once-in-a-generation advantage to her party on the way out of Number Ten, Theresa May will allow the Boundary Commission to proceed with its work of cutting the number of constituencies from 650 to 600.

A full-on offensive to replace moderates with true-believing Corbynites will be too great for Momentum to resist. The resulting schism with the party’s moderate wing will cripple the British centre-left for a generation.

Can Momentum resist the urge to dominate and control?