Ancient Greece poleis (city-states) developed a style of fighting called the hoplite phalanx (which I’ve explained in some detail here). The phalanx reinforced the principle of community identity because it required all the members of the phalanx to stand close together (basically, shoulder-to-shoulder) and advance in unison. Each man’s shield covered half his body and half the body of the man to his left, so to survive required each man to stay close to his neighbors and to fight to keep him alive. Indeed, a phalanx typically lost its battle if a hole opened in its formation.
Additionally, the citizens of a polis were its soldiers. Rather than fielding professional armies of full-time soldiers, most poleis required all their adult citizens to fight when necessary. So these armies were more like the US National Guard than the US Army; the soldiers had other occupations (typically, farming) and were part-time warriors when necessary; they were called up for a battle or a campaign, served without pay, and then demobilized and returned to their normal occupations. To ensure readiness, citizens were generally required to own their own weapons and armor and to meet occasionally to drill the fighting techniques of the phalanx.
The Spartan system was much the same, except that all citizen men were required to be full-time soldiers (with serfs doing the farm-work that other Greeks did themselves). Their culture required them to drill regularly in preparation for war. So while Spartan soldiers were essentially professional soldiers (though not paid professionals), their military system still emphasized communal identity. In neither system was there much room for the individual to act on his own, because doing so would have disrupted the phalanx.
Trireme warfare extended this principle to the seas. Greek Triremes (which I explain here) required hundreds of men rowing in perfect unison; failure to maintain unison would result in tangled oars and the ship being motionless in the water. This required sailors to practice unison rowing. In Athens, the only major difference between serving in a phalanx and serving in trireme (from an organizational standpoint, that is) is that rowers were paid a daily wage for their service, thus guaranteeing that the rowers would not be financially ruined by their service. For the Athenians, their naval system was an expression of their democratic principles, one they were quite proud of.
But in 300 and 300 2: Rise on an Empire, the heroes do not fight in unison or formation, even when the result is complete nonsense (in the case of the battle of Thermopylae). Instead, the various characters (mostly the heroic Greeks) fight as individuals. They rarely make any effort at remaining in formation, assisting each other, or in any way depending on each other (although Leonidas’ suicidal attempt to kill Xerxes does require someone for him to use as a trampoline, something I suspect the Spartans would have found deeply insulting).
The result is warriors who win their fights out of sheer heroic bravery and the fact that they are the good guys. They win because they try hard and really care about their cause, rather than because they are actually skilled at what they’re doing. Their skill is to a considerable extent an expression of their moral character, and the lack of skill (the simple killability) of the bad guys is a reflection of their essentially immoral nature. In both movies, there’s only one bad guy who actually exhibits any true combat ability, and that’s Artemisia, who not coincidentally is also the only bad guy we’re encouraged to empathize with to any degree at all.
This, of course, is an example of the American tradition of Heroic Individualism that is so powerful in modern cinema; 300’s Spartans have a great deal in common with the cowboys of many Westerns. This is not entirely anachronistic, since the heroes of classical Greek literature are also Heroic Individuals; Achilles in the Iliad fights much the way that Zack Snyder’s Leonidas does, and with the same ultimate consequence (although Achilles proves more capable of learning from his mistakes than Leonidas does).
But while the Greeks might have loved the Homeric heroes, they recognized that they could not (or perhaps could no longer) fight that way. They developed a system of fighting that required reliance on community and mutual support and which actively had to rein in the individual (in fact, Roman soldiers could actually be punished for trying to be too heroic, because it usually led to them being separated from their phalanx and having to be rescued).
The result of all this is that 300 and 300 2 are unable to tell their stories coherently. Neither film makes much sense because they are unable to reconcile the events they are trying to depict with the need to make their characters Heroic Individuals. As I’ve said, Snyder’s depiction of the Battle of Thermopylae doesn’t even make sense on its own terms, and Murro’s depiction of the naval side of the 3rd Persian War is pure fantasy with incongruous details like cliffs that appear out of nowhere and a general who rides his horse across the sea. In a way, it’s an indictment of the entire ethos of Heroic Individuality that its conventions can’t be merged with historical events in a way that fully makes sense. But then, no one went to see either of these movies hoping to see sensible depictions of anything, did they?