Hello there!

Great question, and honestly it's a very loaded question. A huge chunk of the historiography around the French Revolution for the past 200+ years has been devoted to the question of why the Revolution became as radical as it did; why the Revolution of 1789 turned into the Revolution of 1792. The question comes down in large part to the "school of thought" that a Historian aligns with, which is in large part shaped by their time-period and their views on politics, economics, and human nature. However in these debates over the origin of this 'radical' period in the French Revolution there are some events that we can identify as important in explaining the shift, and that most historical ideologies would agree with-- though their stack-rankings would be different given their point of view.

First I'd like to start with the obvious: As with all things in History, there is not one cause, one reason, one event that forced the events to go from reform to revolution. Instead there was a complex interplay of many different events, personalities, and decisions that shifted the movement into a more radical direction.

The second quick caveat I'd like to put out there is we often talk about historical events in discrete time periods that belies the true impact that they had on the lives of those involved both before and after these finite endpoints. Though (most) say the French Revolution "began" in 1789, it's important to know that political unrest had been ramping up for about 40 years at that point, with dissatisfaction with the monarchy dating back even further. My point here is that while it might feel "sudden" to those of us studying the Revolution, this about-face on the monarchy certainly wasn't as sudden as we sometimes think, as we visualize it as a two-year turnaround rather than an event that had been building for decades.

Okay, onto the Revolution itself. There are a few events that I think deserve special mention to explain why the tenor of debate went from "how to make existing government and social structure better" to "nah screw this it's Republic time". In no particular order they are Louis XVI's (who he was as a ruler), his disastrous Flight to Varennes, and the War with Austria and Prussia. Now entire books are written about each of these subjects individually, but I will try to give a brief overview of why and how each of these contributed to the radicalization of the Revolution.

First let's start with Louis himself. I think Mike Duncan summed him up very well in his Revolutions podcast on the French Revolution (highly recommend) "Louis wasn't a bad man, and he wasn't even a particularly a bad king. He was however a terrible crisis manager, in what could be considered one of the world's worst crisis." The old cliché that Louis could never make up him mind about anything does seem to be true-- at least in part. While in private it does appear Louis had a clear vision for how he wanted things to run, (and surprisingly his personal views seem more liberal than you'd imagine, as he was an avid Enlightenment reader), in public Louis just failed to commit. When things got tough, when Louis ran into resistance, he tended to fold. I think the best way to describe Louis was a people-pleaser. He wanted to be liked, he wanted to 'do the right thing', but by trying to please everyone he wound up jumping back and forth between different positions. This vacilation entirely destablized the regime in a time when the monarchy desperately needed a rock to hold the nation together. We see periods throughout the early Revolution, what could be termed the "Constitutional Monarchy" phase of the Revolution, where Louis seems ready and willing to be a Citizen King-- and the people seem overjoyed to accept him as such. But Louis just couldn't decide which way he wanted to go. Louis does things like refuse to sign the Declaration of the Rights of Man and of the Citizen, even after he said he would. Then the Women's March on Versailles in October 1789 forces him to do so as part of a larger capitulation... so did he really want to, or was his hand forced? Then you the great battle over the royal veto-- a huge turning point in the struggle for power between the right and left in the National Assembly. The conservatives were pushing hard for a full, unequivocal royal veto, which would have given Louis essentially the power of an absolute monarch. The left on the other hand was pushing for no veto whatsoever, which would have relegated Louis to figurehead status. The battle was by no means decided, and indeed those in the middle, spooked by recent popular unrest, seemed to be moving towards the right rather than the left, when out of no where Louis steps in and lets them know he'd be open to a compromise whereby he gets a veto, but there are strings attached (such as having to wait multiple legislative sessions in order to wield it, severly lessening its effectiveness). After that, the right could hardly continue to champion for a full veto when the king himself was saying he was open to a partial veto, and so this compromise won the day. Finally we have what appeared to many at the time to be the proof of Louis's commitment to the Revolution: the Fete de la Federation, the great festival held on the Champs de Mars in 1790 to celebrate the anniversary of the Fall of the Bastille. Amid enormous fanfare, in a solemn ceremony, Louis swore to uphold the still-in-the-works Constitution (to be called the "Constitution of 1791") and to protect the gains of the Revolution. He was no longer Louis XVI, King of France and Navarre, but Louis XVI, King of the French. This honeymoon period didn't last for long. Though the Fete helped paper over the cracks for a while (like going on a fun vacation with a spouse you've recently been fighting with), thigns quickly resumed their hot-and-cold nature. Louis, and particularly his Queen Marie-Antoinette, began to walk back some promises they had made, and didn't act quite like devoted monarchs of a beloved people, but more like prisoners wishing to escape the masses. A poignant scene was when the royal family tried to leave for their chateau in Saint Cloud, a suburb outside of Paris, for the Easter holiday in 1791 and were prevented from leaving by suspicious crowds. It is said that this was about the time Louis & Marie-Antoinette began to plan, with the help of loyal courtiers, their comedy of errors escape attempt, the event known to posterity as the Flight to Varennes.

Now I place the Flight to Varennes as a cause of radicalization all its own due to the fact that it was more than an extension of Louis just being an impotent king. The Flight to Varennes appears to be an event where those of us looking back can draw a pretty definied line in the sand, and say "before 20 June 1791, tense, shaky willingness to accept and work with the king. After 21 June 1791, widespread rejection of the king". Like the saying goes, trust takes years to build, and seconds to demolish. Well in making the decision to flee Paris in the middle of the night on 20 June, 1791, Louis completely shred up whatever good will and trust he still had with the people of France. As someone who has studied this radicalization of the French Revolution extensively, I find it very compelling to identify the Flight to Varennes as the point when radicalization began, though there are certainly other points of view here that others find more fruitful. If anyone is interested further, this is not an original idea of mine, but rather an argument to be found in the fantastic book When the King Took Flight by historian Timothy Tackett. I highly recommend the book, not only for how fascinating the subject matter is, but because Dr. Tackett manages to write good, solid history in a way that is accessible and interesting to those new to the subject. For those not familiar with the Flight to Varennes, it was the royal family's attempt to escape from Paris in the middle of the night (yes, costumes were involved) and flee to Montmedey, a fortress on the Austrian border. The idea was to meet up with a loyal army garrison, and to ???. We can only speculate, but given correspondence that was found after the King was hauled back to Paris and taken prisoner it is clear that the King wished to "restore order", and that this "order" would have taken the form of an almost wholesale repudiation of the Revolution he had so recently sworn to uphold. As it was the King was caught in the town of Varennes, and hauled ignobly back to Paris.

(Continued in comment below!)

I am not a specialist of the French Revolution, but there are a few things that can explain the failure of the reforms taken by the National Assembly between 1789 and 1792, and the radicalization of the revolutionary movement.

(1) A financial crisis. The calling of the Estates General in 1789 was rendered necessary by the need for financial reforms in the Kingdom. The State was heavily indebted, while incapable of developing a functional taxation system. This led to two major crisis : one monetary as there was shortages of silver and coins, one inflationary as the grain supply was used both as a tool to refinance the state and as a way for speculators to make profit while the harvests of both 1788 and 1789 were disastrous (blame climate). These problems were neither solved by the King nor the National Assembly. In fact, they lasted during the whole Revolution, even into the reign of Napoleon, and they kept putting people (and mostly the citizens of Paris) on the brink of despair and prone to take the streets and ask for more radical actions to be taken.