Planck had always been a strong supporter of his country, taking great pride in the fact that he was German. Above all else, he valued unity, which to him meant both unity between scientific disciplines and unity between the German people. For this reason, Planck was enthusiastic at the start of World War I, because he saw it as an opportunity for the German people to pursue the noble purpose of national unity.
Consumed by patriotism, and believing that Germany was only defending itself, Planck agreed to sign a manifesto in support of the German cause, known as the Appeal to the Cultured Peoples of the World. Also known as the Appeal of the Ninety-Three Intellectuals, it appeared in all the major German newspapers and in other papers across Europe. Many of the signers, including Planck, signed the manifesto before they had read it, and because of its strong language and the wildly negative reaction it received from the rest of the world, many of these signers soon came to regret their haste.
For the first year of the war, Planck remained convinced that Germany was in the right. Although the war drew many students and teachers away from the schools–even Planck's children took part in the war effort–Planck felt the loss was a necessary sacrifice. He waited patiently for the world to understand that Germany would emerge triumphant. In a letter to a colleague, he noted, "Besides much that is horrible, there is also much that is unexpectedly great and beautiful It is a great feeling to be able to call oneself a German."
However, by 1915, Planck's actions had become somewhat more moderate, either because he believed Germany had finally gone too far or because he suddenly considered the possibility that Germany might not win. Either way, over the next year, he became a public champion of moderation within the German physics community. In 1915, Planck refused to sign a friends' manifesto that claimed that German physicists were enemies of British physicists and should have nothing to do with them or their journals. Privately, Planck thought this might be a principled stance to take, but he knew it would become problematic if Germany lost the war.
Then in 1916, he went semi-public with his regret about having signed the Appeal of the Ninety-Three Intellectuals. In a letter to another physicist, published with Planck's permission, Planck explained that he was still a staunchly patriotic German, but he had nothing but respect for his colleagues in enemy countries.
The war ended a few years later, but Planck did not emerge unscathed. Within a year after the war, all but one of his four children were dead. His son Karl died from injuries received in battle. His daughter Grete died in 1917, one week after childbirth. And Grete's twin sister Emma, who had married Grete's widower in January of 1919, died before the year was over.