Achilles heel

In Greek mythology, Achilles is made invunerable by being dipped in the river Styx by his mother. However, his mother held him by the heel, and eventually Achilles was felled by an arrow that hit this unprotected part.

Today the term Achilles' heel refers to any inherent weakness. For instance, the term is being used to describe the US's inability to defend itself from weapons of mass destruction. While their military is able to fight any other military with relative ease, it is somewhat ill-equipped to fight against small terrorist actions: this is the "Achilles heel" of the US military.