There are various reasons that contributed to the defeat of Persia by Athens in the period of 490BC-479BC. These included unity and leadership, strategy and tactists, superor Greek weapons and armour, terrain and lack of planning. All these factors were major contributors and therefore it can be said that there was not just one reason that led to the fall of the Persian Empire.
Strategy and tactists were one of the reasons that contributed to the defeat of the Persian Empire. Greek Strategies were used for larger reasons to the Persian strategies. The Persian troops were not trained which didn’t match the phalanx formation of the Ancient Greek hoplites. Athenians attacked Persians on the beach. The Greeks extended their group that reinforced the flanks and thinned their centre and they commentator the edges along terrain obstacles. Persians pushed through the weakened Greek center. The strong Greek edges destroyed the lightly armed Persian sides. This therefore caused the defeat of Persian as it collapsed on the Persian centre.

Lacks of planning was another reason why the Persians got defeated. The Persian armies were larger but were disorganised. This meant that communication between the troops were often a problem. The Persian recruits often panicked, especially when their leaders were killed.
The Persian infantry had no defensive weapons. The Greek hoplites soldiers were most advanced in the world in the time frame of 490BC-479BC. Bronze armour and shields were used and very hard targets.

Strategic
By land, supported logistically by the navy.
Persian strength - Army
Not limited on the number of troops
Long, difficult, time consuming route.
By sea, with an amphibious landing.
Direct route
Persia is not a naval power.
Size of invasion force limited by transport capacity.

492 B.C. - Darius attacks by land, supported logistically by the Navy.
Movement halted when the navy is destroyed in a storm off of Mt Athos.

Tactical
Hand-to-hand combat = decisive victory for Athenians

The Persians were superior in their cavalry but the Greeks overcame through clever tactics and by choosing