The phalanx was a military formation used by the ancient Greeks and was particularly effective during the time of Alexander the Great. However, after Alexander's death, the phalanx declined in use for a few reasons.

Firstly, the phalanx was heavily reliant on the sarissa, a long spear that was over 18 feet in length. The sarissa was a difficult weapon to use, and its length made it unwieldy and difficult to maneuver. As a result, it required extensive training to master, and this made it difficult for armies to replace losses in trained soldiers.

Secondly, the phalanx was primarily a defensive formation, and it was less effective in offensive operations. Alexander's army had used a combination of phalanx and cavalry to great effect, but after his death, this balance was upset, and armies increasingly relied on cavalry to achieve victories in the battlefield.

Thirdly, the decline of the Greek city-states and the rise of the Hellenistic kingdoms led to changes in military tactics and strategy. The Hellenistic armies placed greater emphasis on siege warfare and used a range of new weapons, including ballistae and catapults, that were not compatible with the phalanx formation.

As the use of the phalanx declined, armies began to rely more on professional soldiers and mercenaries. These troops were often armed with shorter weapons, such as the gladius or the kopis, which were more suitable for close combat.

In summary, the decline of the phalanx can be attributed to its limitations in offensive operations, the difficulty in training soldiers to use the sarissa, and the rise of new weapons and tactics in the Hellenistic period. It was replaced by a combination of cavalry and infantry armed with shorter weapons, as well as by professional soldiers and mercenaries.
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