Ancient Athens had several weaknesses that eventually led to its downfall when the Macedonians conquered it in the 330s B.C.E.
One of its weaknesses was the democratic system itself. Often praised for being the foundation of democracy in the West, Athens had a direct democracy. This meant that all Athenian men could vote, and the victor was decided by a majority. In addition, the representatives were chosen from the population and would serve a one-year term. However, it is important to note that only Athenian males over 18 could engage in the democratic process; women, slaves, and non-Athenians had no political power. One of the concerns of the one-year term limit was that people could make bad decisions or did not have enough time in office to make a lasting impact. Over time, we do see this happen with disastrous results, like with the Peloponnesian War. Famed general Themistocles, who helped lead the Athenians to victory during the Persian War, was exiled from Athens and was not present to help Athens during the Peloponnesian War, which expedited their downfall.
Another Athenian weakness was their choices during battle during the aforementioned Peloponnesian War. Known as excellent sailors, the Athenians were powerless against the far superior fighting forces of the Spartans. In order to combat this weakness, the Athenians built a wall surrounding their city and their nearest port, Piraeus. Being confined in the walls for so long, disease quickly struck, and a plague wiped out a large portion of the population. Among those who died was the general Pericles, and with his death, Athens lost its greatest chance at defeating the Spartans. Several other events led to Athenian downfall, such as the disastrous Sicilian Expedition, where the Athenians attempted to get Spartan ally Sicily to turn against their Spartan friends but failed.