Although Louis XIV, the Sun King, expanded France's territory, he also suffered setbacks.
Louis XIV's mother was Anna of Austria, and this Austrian princess served as regent before he ascended the throne.
That was the relationship between France and Austria: they intermarried while remaining wary of each other. The fortress system Louis XIV built along the border of Flanders once saved Paris during the French Revolution. Vauban, the engineer of the time, not only built fortresses but also used forests, terrain, and other conditions to conceal sluice gates, expanding the floodwaters and turning the entire region into a floodplain.
It was somewhat like artificially building a Yellow River. All it took was opening the sluice gates to send floodwaters rushing into the mountain passes. Fifty thousand National Guard troops would be enough; there would not even be a need to deploy regular troops. It had cost five hundred million francs at the time, and if its construction period was one hundred years, four million francs had to be spent on maintenance every year.
During his first Italian campaign, Napoleon set out from Savona and crossed the mountains at the weakest point where the Alps and the Apennine Mountains connected, cutting off the Austrian and Sardinian armies and seizing the fortress at the confluence of the Tanaro River and the Stura River.
Fortresses were difficult to take, but the rewards after capturing one were immense. With that foothold, he established his own depots and forced the King of Sardinia to hand over the fortress of Tortona, which lay in the direction of Milan.
An expeditionary army needed supplies. Along the long supply lines, the fortresses contained depots and hospitals. Those guarding them were recovering sick soldiers and recruits, while the men who followed him on forced marches were healthy, strong soldiers.