To write notes on 20th-century war and peace, you can take the following advice: 1 Understand the wars of the 20th century, including the First World War, the Second World War, the Cold War, and the Vietnam War. You can read books related to the history of war, such as The Art of War, War and Peace, and other classic literary works to understand the cruelty and complexity of war. 2. Study the peaceful events of the 20th century, such as the establishment of the United Nations, the collapse of the bi-polar structure, the wars in Iraq, and the wars in afghana. You can read related books on the history of peace, such as the United Nations Constitution, Introduction to International Law, and other basic knowledge books to understand the theory and practice of peace. 3. Read relevant historical documents and news reports such as historical books, newspapers, magazines, television and radio to understand the background, influence and development of war and peace events in the 20th century.