Page "Battle of Marathon" Paragraph 23

The second theory is simply that the battle occurred because the Persians finally moved to attack the Athenians.

Although this theory has the Persians moving to the strategic offensive, this can be reconciled with the traditional account of the Athenians attacking the Persians by assuming that, seeing the Persians advancing, the Athenians took the tactical offensive, and attacked them.

Obviously, it cannot be firmly established which theory ( if either ) is correct.

However, both theories imply that there was some kind of Persian activity which occurred on or about the fifth day which ultimately triggered the battle.