After Frederick the Great’s death in 1786, the throne was inherited by his nephew Frederick Wilhelm II. , who wasn’t as interested in the army’s well-being as his uncle was. He left the army’s management to the Duke of Brunswick, Charles William Ferdinand. The Duke was an old man with an old approach to the arts of war. He preserved the same drilling methods and the same equipment used by Frederick the Great decades ago. As a result, army officials and soldiers were not efficiently prepared to fight against forces with modern training and equipment. Most of the Prussian nobility and generals, however, were still believing that the “superiority” of the Prussian army was proved by Frederick’s campaigns and no reforms were required.
In 1792, the French Revolution was growing with a rapid pace. The King was unofficially detained and the monarchies of Europe were beginning to get concerned that their thrones would be the next. The Austrian emperor, who was the brother of the French queen Marie Antoinette ordered his troops to rally towards the French border. The revolutionary government demanded the Austrian troops to be removed from the border, and as soon as the negative reply came, France declared war on Austria on 20 April 1792. The revolutionary armies hastily invaded the Netherlands, which was under Austrian sovereignty, thinking that the Dutch would join their side against the Austrians. These expectations slowly faded away as the Dutch didn’t revolt and the revolutionary army’s organization began to degrade terribly. Prussia joined the war on Austria’s side on 28 April and marched its armies to the French border. Other German states of the Holy Roman Empire also joined the coalition.
On 25 July, Commander of the Prussian army, the Duke of Brunswick declared a manifesto, threatening the Revolutionary government that no harm should be done to the King and his family or else the Coalition armies would march onto Paris. The manifesto drastically helped the French government in their efforts to gain war support from the public. Many citizens took up arms upon hearing about the manifesto. The coalition armies began moving into French territory and at first didn’t meet with strong resistance. They captured important forts such as Verdun in a few weeks. The revolutionary army, led by Charles François Kellermann, finally faced the coalition army, led by Duke of Brunswick, at Valmy. The outcome of the battle was a great victory for the revolutionaries as they managed to repel the invasion with a fairly disorganized and numerically inferior army which the enemy saw as a “group of bandits”. Prussian armies, even though they didn’t lose fighting strength, decided to retreat outside the French border as the war was starting to look a lot more costly and difficult to wage. The failure of invasion against a disorganized army raised questions about the actual effectiveness and fighting strength of the army between Prussian officials.
After their victory in the Battle of Valmy, the Revolutionary government formally abolished monarchy and declared the First Republic on 21 September. The French armies went on an offensive after seeing the Prussian retreat and gained territories along the Rhine and defeated the Austrians in the Netherlands. The French aggression caused Britain, Spain and Portugal to join the war in early 1793. Prussian army besieged a large French garrison in the city of Mainz in April and captured around 20.000 French soldiers at the end of the siege. While the French was fighting in many fronts, Prussia wasn’t able to afford many troops as the state was on the verge of bankruptcy. Another significant battle fought by the Prussian army was the Battle of Pirmasens, at which the Prussian army defeated the numerically superior French troops. Towards the end of 1793, the French had already lost many of the lands they occupied earlier.
Tides turned in 1794, however, as the French draft policy started to prove useful as they were placing too many troops on the battlefield for the monarchies to compete with. They managed to advance in almost all fronts and Prussia was too weak to respond with a decisive counter attack in the Rhineland front. In early 1795, the Dutch Republic collapsed to French offensive and Prussia decided to call for a peace on its behalf. The French Republic and the Kingdom of Prussia signed the Peace of Basel on 6 April. According to the treaty, Prussia recognized French sovereignty over the west of the Rhine and in return, France left the territories it occupied on the east side of the Rhine.
With the treaty, Prussia officially left the First Coalition and remained loyal to this treaty until 1806 when it joined the Fourth Coalition against Napoleon. The disastrous defeats of the Revolutionary Wars allowed the birth of reform movements in Prussia, these movements undertook the duty of reorganization of the armies and were especially a key factor in Prussia’s resurgence after the catastrophe of the Fourth Coalition.