My understanding is that reactions to both regicides were (broadly) of shock/horror for much of European society- other than a minority of a more radical persuasion. However, the reaction to the French Revolution and the execution of Louis XVI seems to be rather more heavy-handed and outright condemnatory than to the English Civil War and the execution of Charles I. Indeed, many European powers declared war against the new French Republic over the radical outcome, yet there were no similar actions against the nations of Britain.
Firstly, is this view broadly accurate? If so, why was this the case?
Hello! I'll be sticking mostly to the French Revolutionary side of this question, but I believe it really will provide a pretty satisfying answer to your question overall!
Where the confusion comes from is the popular misconception that the nations of Europe declared war on Revolutionary France after and therefore because they beheaded Louis XVI. Actually, Mr. Louis himself (using his power granted to him by the Constitution of 1791!) declared war on Austria back in April of 1792--almost a full year before he was guillotined. Therefore the nations of Europe were already at war with France when the King's head was on his body & his butt on the throne.
The lead-up to war in 1792 was a bit like a rorschach test: every Frenchmen could look at the proposed war with Austria and Prussia and see what they wanted from it, thereby giving it their utmost support, applauding & cheering for la nation elbow-to-elbow with a guy doing the same, but for the diametrically opposite reason.
War fevor had infected virtually everyone in power come 1792: the *Girondins--*or "Brissotins" at this point, so-called after the tireless voice for war leading the Legislative Assembly, Jaques-Pierre Brissot--were a liberal party looking to secure the government within their own possession, cementing the Revolution and winning honor for the patrie under the leadership of the Third; the liberal nobles, recently fallen from grace, sought the war to regain a bit of former pomp & circumstance (LaFayette); the arch-conservative émigrés outside of France wanted to ride triumphant back into the promised land, putting down the heathen dogs and reclaiming their birthright, while the King and Queen wished from within France very much the same.
The liberal factions, whatever their reasons for their warhawking, knew that France would secure a great and glorious victory against her enermies; the conservatives were certain of the opposite.
At this point I am contractually obligated to mention that there were two bros bold enough to stand up to a nationwide push to war: Robespierre, and Danton. Both agreed that to push for an aggressive war would be insanity--we're literally undergoing a massive revolution, and now is a good idea to go to war? It remains a valid point. Robespierre & Danton, standing together against the masses....the good old days....
Now why war with Austria & Prussia specifically? Meh, a little bit of proximity, a little bit of a hated Austrian-French alliance boiling over into hate after a few decades of simmering resentment (plus a dash of spite for the "Austrian She-Wolf" Marie-Antionette); the fact that the German principalities had--at least initially--harbored the émigrés and hosted their court in exile; and then just general saber-rattling by all involved. For the Austrians and Prussians, the chest-thumping was a show of dignified force against the beligured French army, decimated by deserting noble officers and headless left; but the honor of Leopold's sister (later Francis's aunt) Marie-Antionette factored into at least the optics. The French did not thump bellicose so much as patriotically bare their chests in a emotional fervor, glorifying in the rights of man, the honor of la nation, and the intoxicating new bonds of fraternité that spurred men on to great and herioc ideas of honor defended. Except Robespieere & Danton.
War was not inevitable, and it certainly was linked to the Revolution, but it was not a response to a despot dethroned. Rather, it was a continuation of hostilities that had ebbed and flowed starting with the Sun King and smoldering since the Seven Years War. Contrast this to the English Civil War, a far more parochial affair. The wars were there, but there were confined for the most part on the islands--the channel standing as barrier from contagion. From an ideological perspective, however, there was also an important difference. While the early French Revolution was certainly not the exhaultion of the artisan in the Terror of '93-'94, there were the lanterns of the Enlightenment, casting an albeit dim light over the formerly obscure, and for the first time exposing even the commonest paysan to the natural rights of man and of the citizen. These ideas weren't yet ripe on the vine yet for the English Civil War--John Locke grew up in their shadow--and therefore when unrest burst upon France a century later, it was no longer "Civil" infighting amongst the holders of power, deploying a gentlemanly charge of cavalry--but Revolution, fought from the bottom, up, with pikes in hand. And the pike probably has a head on top of it.
Please let me know if you have any more questions!