Westheimer joined the Haganah Jewish Zionist underground paramilitary organization (later, the Israel Defense Forces) in Jerusalem. Because of her diminutive height of , she was trained as a scout and sniper. Of this experience, she said, "I never killed anybody, but I know how to throw hand grenades and shoot." She became an ace sniper, and learned to assemble a rifle in the dark. When she was 90 years old, she demonstrated that she was still able to put together a Sten gun with her eyes closed.
