In order to deal with the growing financial crisis engulfing France, King Louis XVI summoned the Estates-General. That the Estates-General had not been summoned for a 175 years gives you some idea of how dire the situation was at this point.
In 1789, France was on the brink of bankruptcy. Previous attempts at financial reform had met with little success, not least due to the overwhelming hostility of the First and Second Estates, the clergy and the nobility, respectively. To make matters worse, France had suffered a string of failed harvests, leading to widespread unrest among the rural population.
Under the circumstances, Louis had very little choice but to convene a grand meeting of the Estates. But despite the urgency of the situation, it soon became clear that the First and Second Estates were neither willing nor able to rise to the challenge of the hour. Desperate to hang on to their tax privileges and contemptuous of the Third Estate, they pointedly refused to work with those they regarded as their social inferiors.
But the arrogance of the First and Second Estates soon backfired. They underestimated the cohesiveness and determination of the Third Estate, whose leaders made it clear that they would conduct the nation's affairs with or without the cooperation of the other Estates.
Due to the intransigence of the First and Second Estates, this is precisely what the Third Estate did. In the famous Tennis Court Oath, members of the Third Estate vowed not to separate until a new constitution had been established.
The extraordinary unity and solidarity of the Third Estate had allowed it to outmaneuver the king and the top two Estates. Before long, most of the nobles and clergy had joined with the Third Estate in the National Assembly, the revolutionary body formed by the Third Estate three days before the Tennis Court Oath. Now that the First and Second Estates had joined the National Assembly, it became clear that the Revolution could not be stopped. The only question that remained was precisely how it would all play out.