heel of Achilles

In 1907, President Theodore Roosevelt began building a new Pacific bastion at Pearl Harbor in Hawaii, not in Manila Bay, saying that the Philippines, by then an American colony, is “our heel of Achilles.”

A sample, from "The Heel of Achilles":

The fable represented that the heel of Achilles, the only vulnerable part of his body, because his mother held him by it when she dipped him in the Styx, was the spot on which he received his fatal wound.

If this is the heel of Cornwall, it, like the heel of Achilles, is vulnerable, and nearly severed by the slash!

But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced by the heel of Achilles the laws of Moses the temple of Isis The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.