The International Court of Justice (ICJ) has to decide on whether Israel’s alleged crimes in Gaza amount to a genocide. Key to the ruling will be proving intent. Because my father fled Nazi Germany in June 1938 as a 12-year-old boy, I grew up with a deep appreciation for the evil that governments can do. That is part of what led me to devote my life to defending the human rights standards that help to deter oppression. I took seriously the vow, “never again.” But the determination to prevent such atrocities does not mean that anything goes. Rather, international human...