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Why did tightly packed infantry formations become dangerous after firearms improved?

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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.

Tightly packed infantry formations became increasingly dangerous as firearms technology advanced because they presented a dense, vulnerable target to weapons with rapidly improving range, accuracy, and lethality.1

During the 19th century, the Industrial Revolution transformed small arms through the introduction of rifled barrels, breech-loading mechanisms, and metal cartridges. These innovations allowed soldiers to fire with greater speed and precision from much longer distances. Simultaneously, artillery evolved from firing solid shot to using explosive shells, which significantly increased destructive power.1

When troops remained in traditional, tightly packed formations, these modern weapons turned the battlefield into a site of mass casualties. Tactics that had been standard for centuries became "positively suicidal," forcing soldiers to abandon dense formations in favor of dispersing, taking cover, and lying down to survive. Commanders who failed to adapt to these technological realities—such as during the American Civil War—often subjected their troops to horrific losses. Consequently, the necessity of avoiding concentrated fire led to the dissolution of rigid formations and a shift toward more dispersed, decentralized combat.1