Final answer:
The Spartan army decided to attack Thebes but was driven back during their efforts at the Battle of Thermopylae.
Step-by-step explanation:
The army that decided to attack Thebes but was driven back during their efforts was the Spartan army. In 480 BCE, the Spartans, under the leadership of King Leonidas, made a stand at the mountain pass of Thermopylae against the invading Persian army. Although the Spartans fought bravely, they were eventually surrounded and defeated.