Final answer:
Athens' last effort to defend itself ended with the defeat at the Battle of Aegospotami in 405 BCE, after which it was besieged and surrendered to Sparta in 404 BCE, concluding the Peloponnesian War.
Step-by-step explanation:
Athens made several last efforts to defend itself during various periods of its classical history. One significant attempt came after the disastrous expedition against Syracuse in 413 BCE, where Athens sent thousands of troops in hopes of looting the city, which was a Spartan ally. This expedition failed miserably with the near-total destruction of the Athenian forces. Subsequently, Athens was on the defensive, its military weakened, and its economy in trouble due to a loss of slaves and revenue from its silver mines.
The final and decisive blow occurred in 405 BCE when the new Spartan fleet, funded by Persia, destroyed the Athenian navy at the Battle of Aegospotami. This led to Athens being besieged and eventually forced to surrender in 404 BCE, ending the Peloponnesian War and resulting in the collapse of the Athenian empire.