Final answer:
The phrase should be completed as 'They got money to build a fleet,' which refers to the Spartan construction of a navy funded by Persia to defeat the Athenian naval forces, leading to the end of the Peloponnesian War.
Step-by-step explanation:
They got money to build a fleet which helped the Spartans win the war. During the Peloponnesian War, the Spartans realized they needed to counter the Athenian naval supremacy to defeat Athens and turn the tide of the war.
With funds secured from Persia under the condition that they restore the Greek cities in Ionia to Persian rule, the Spartans built a powerful navy that proved to be decisive. In 405 BCE, at the Battle of Aegospotami, the Spartan fleet, with this new financing, destroyed the Athenian navy, effectively putting an end to the Athenian empire and forcing the city into surrender.
The victory in the naval battle was a critical element in the Spartan strategy, showing the importance of economic resources and strategic military innovation. This allowed Sparta to lay siege to Athens and prevent the arrival of further food or supplies by sea, leading to Athen's capitulation and concluding the drawn-out Peloponnesian War.