Westheimer joined the Haganah Jewish Zionist underground paramilitary organization (later, the Israel Defense Forces) in Jerusalem.[25][24][26] Because of her diminutive height of 4 feet 7 inches (140 cm), she was trained as a scout and sniper.[23][26][27] Of this experience, she said, "I never killed anybody, but I know how to throw hand grenades and shoot."[28] She became an ace sniper, and learned to assemble a rifle in the dark.[23][14] When she was 90 years old, she demonstrated that she was still able to put together a Sten gun with her eyes closed.[29]