Some historians believe that Athens concluded a peace treaty with Persia in 449, two years before work began on the Parthenon. The significance of this would be that the Delian League/Athenian Empire continued to exist, even after the reason for its existence (a mutual defense league against the Persians) had ceased to be valid. In other words it was now openly acknowledged that Athens was not just the head of the Greek defense league but actually an imperial master over other Greek states. The decision by the Athenians in 454 BC to move the League treasury from the Panhellenic sanctuary at Delos to the Athenian acropolis points in the same direction. Because the Parthenon was built with League funds, the building may be read as an expression of the confidence of the Athenians in this newly naked imperialism. But the piety of this undertaking should not be underestimated; the Persians had sacked the temples on the Athenian acropolis in 480, and rebuilding them fulfilled, in Bury's words, the Athenians' "debt of gratitude to heaven for the defeat of the Mede." The