In the aftermath of the invasion and the Destruction of Athens by the Achaemenids, the Athenians began rebuilding their city under the guidance of Themistocles in the autumn of 479 BC. They wished to restore the fortifications of Athens, but the Spartans objected on the grounds that no place north of the Isthmus should be left that the Persians could use as a fortress. Themistocles urged the citizens to build the fortifications as quickly as possible, then went to Sparta as an ambassador to answer the charges levelled by the Spartans. There, he assured them that no building work was on-going, and urged them to send emissaries to Athens to see for themselves. By the time the ambassadors arrived, the Athenians had finished building, and then detained the Spartan ambassadors when they complained about the presence of the fortifications. By delaying in this manner, Themistocles gave the Athenians enough time to fortify the city, and thus ward off any Spartan attack aimed at preventing the re-fortification of Athens. Furthermore, the Spartans were obliged to repatriate Themistocles in order to free their own ambassadors. However, this episode may be seen as the beginning of the Spartan mistrust of Themistocles, which would return to haunt him.