From my article in the October 2018 issue of History Today:
By the early months of 1658, the young English republic was in crisis. Nine years earlier, after a decade of civil war and three years of famine, leaders in the New Model Army had, as the poet Andrew Marvell later put it, ruined ‘the great work of time, / And cast the kingdom old / Into another mould’. In early December 1648, frustrated by the ambiguities and hesitations of those negotiating with the defeated king, a detachment of soldiers purged Parliament of its most conservative MPs, opening the way for the hasty organisation of a High Court of Justice and the consequent trial of Charles I. As the king resisted the legitimacy of the court and refused to enter a plea, power passed to the more radical military and political leaders, as they became increasingly interested in exploring republican solutions to the political crisis.
You can read the whole article here.