shown at the Battle of Magnesia, where, once the Seleucid supporting cavalry elements were driven off, the phalanx was static and unable to go on the offensive against its Roman opponents (although they continued to resist stoutly and attempted a fighting withdrawal under a hail of Roman missiles, until the elephants posted on their flanks panicked and disrupted their formation). The Macedonian phalanx could also lose its cohesion without proper coordination or while moving through broken terrain; doing so could create gaps between individual blocks/syntagmata, or could prevent a solid front within those sub-units as well, causing other sections of the line to bunch up. In this event, as in the battles of Cynoscephalae and Pydna, the phalanx became vulnerable to attacks by more flexible unitssuch as Roman legionary centuries, which were able to avoid the sarissae and engage in hand-to-hand combat with the phalangites. Another important area that must be considered concerns the psychological tendencies of the hoplites. Because the strength of a phalanx depended on the ability of the hoplites to maintain their frontline, it was crucial that a phalanx be able to quickly and efficiently replace fallen soldiers in the front ranks. If a phalanx failed to do this in a structured manner, the opposing phalanx would have an opportunity to breach the line which, many times, would lead to a quick defeat. This then implies that the hoplites ranks closer to the front must be mentally prepared to replace their fallen comrade and adapt to his new position without disrupting the structure of the frontline. Finally, most of the phalanx-centric armies tended to lack supporting echelons behind the main line of battle. This meant that breaking through the line of battle or compromising one of its flanks often ensured victory. Classical decline and post-classical use After reaching its zenith in the conquests of Alexander the Great, the phalanx began a slow decline, as Macedonian successor states declined. The combined arms tactics used by Alexander and his father were gradually replaced by a return to the simpler frontal charge tactics of the hoplite phalanx. The expense of the supporting arms and cavalry, and the widespread use of mercenaries, caused the Diadochi to rely on phalanx vs. phalanx tactics during the Wars of the Diadochi. The decline of the Diadochi and the phalanx was linked with the rise of Rome and the Roman legions from the 3rd century