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Why did early modern states build belts of fortified cities along their frontiers?

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tactics Sumerian phalanx, a block of foot soldiers depicted standing shield-to-shield, presenting spears, and advancing in a dense mass, detail from the Stele of the Vultures, limestone bas-relief, c. 2450 bce; in the Louvre Museum, Paris.

During the early modern period, the development of gunpowder and artillery initially threatened the effectiveness of traditional medieval defenses. However, the invention of geometric fortifications, known as the trace italienne, restored the defensive balance by creating structures capable of withstanding bombardment.1

Strategists in this era utilized these advanced designs to construct belts of fortified cities along a country’s frontier as a key component of their peacetime planning. These fortified cities served as powerful obstacles to enemy movement, forcing invading armies to engage in time-consuming and difficult sieges. By creating these defensive zones, states could effectively control territory and protect their borders against aggression, as these star-shaped fortresses—often featuring earthen embankments—were virtually impregnable to the assault weapons of the time. This approach transformed warfare into a more methodical, scientific endeavor where engineering and geometry played a central role in maintaining state security.12