This is not the same thing as believing he was undertaking a suicide mission. Leonidas had no reason to believe that the force he took north was not sufficient to hold the Pass until Sparta and other cities, the Karneia and the Olympic Games over, could deploy their main forces. Leonidas did not, after all, march north with just 300 men. In addition to the Spartiates, he had perioikoi troops, allies from the Peloponnesian League, Thespians, Thebans and Phocians. Leonidas had between 6,000 and 7,000 Greek hoplites at Thermopylae, a pass that at that time narrowed down to a cart track at two places.
To be sure, Leonidas allegedly knew from the Delphic oracle that his own fate was sealed. He presumably expected to die, but there was no reason to assume his death would come soon or that it would be futile. On the contrary, Delphi had promised to save Sparta, if one of her kings fell in battle. Leonidas most likely believed (or wanted to believe) that although he would die, his army would be successful. Nor did he expect all the Spartiates he took with him to die. The fact that he took only the fathers of living sons north with him was not because he expected them all to die, but because he expected some of them would die. He did not want to risk the extermination of even a single Spartiate family – not when he had so many men to choose from.
Otherwise, Leonidas appears to have developed a highly effective strategy for defending the Pass, one that effectively neutralized the superiority of numbers on the Persian side and enabled a comparatively small number of defenders to hold the overwhelming might of Xerxes' army for two full days. Although – or rather because - Herodotus does not give us the casualties of the first two days, we can presume that they were not inordinate. The strategy of defending the “Middle Gate,” which was wider than the “Eastern” or “Western Gates,” appears to have given the Greeks the optimal opportunity to reduce Persian pressure while enabling them to bring sufficient numbers of their own troops to bear.
Significantly, Leondias evidently welded the different contingents together and succeeded in getting them to cooperate. Herodotus says that the allies fought in relays, or turns, so that the troops from each city had time to rest, refresh themselves and tend their wounds between taking their turn at the front. While this sounds logical and reasonable, it is far from self-evident. It would also have required considerable skill in execution – or each change would have produced confusion that the Persians could have exploited.
Equally impressive is Leonidas' performance on learning that the Persians were not only on the trail around his position but the Phocians had already been routed without even sending for reinforcements. This news must have been a horrible blow to Leonidas, one he could not have expected. Yet the surviving accounts indicate that he did not panic, but made a difficult but rational decision to send the bulk of the troops out of the pass to safety so they could "live to fight another day," while retaining a force large enough to delay the Persians long enough for the bulk of the army to get away.
Leondias' stand at Thermopylae on the Third Day of battle was certainly a suicide mission, but not a senseless one. It was no different than the stand of the Brigadier Claude Nicholson with the 3rd Royal Tank Regiment and the Queen Victoria Rifles at Calais in 1940. Nicholson defended Calais on the orders of Prime Minister Churchill "to the last" in order to pin down the Wehrmach and enable the evacuation of the bulk of the British Expeditionary Force at Dunkirk. Both Calais 1940 and Thermopylae 480 were military operations of vital importance to the survival of the nations the sacrificed troops were defending.
Again Leonidas showed his competence as a military leader precisely in the fact that even in a hopeless situation he was able to inflict terrible losses on the enemy and maintain the morale of his own troops. WIth the exception of some -- but not all -- Thebans, Leonidas' men fought to the death. And they fought for him. There can be no greater tribute to a military leader.