In July 1914—shortly before the outbreak of World War I in August—Hahn was called to active duty with the army in a Landwehr regiment. Meitner undertook X-ray technician training, and a course on anatomy at the city hospital in Lichterfelde. Meanwhile, she completed both the work on the beta ray spectrum that she had begun before the war with Hahn and Baeyer, and her own study of the uranium decay chain. In July 1915, she returned to Vienna, where she joined the Austrian Army as an X-ray nurse-technician. Her unit was soon deployed to the Eastern front in Poland, and she also served on the Italian front for a while before being discharged in September 1916.
Meitner returned to the KWI for Chemistry and her research in October. In January 1917, she was appointed the head of her own physics section. The Hahn-Meitner Laboratory was divided into separate Hahn and Meitner Laboratories, and her pay was increased to 4,000 marks. Hahn returned to Berlin on leave, and they discussed another loose end from their pre-war work: the search for the mother isotope of actinium. According to the radioactive displacement law of Fajans and Soddy, this had to be an isotope of the undiscovered element 91 on the periodic table that lay between thorium and uranium. Kasimir Fajans and Oswald Helmuth Göhring discovered this element in 1913, and named it brevium after its short half-life. However, the isotope they had found was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope of the same element.
In 1914 Hahn and Meitner had developed a new technique for separating the tantalum group from pitchblende, which they hoped would speed the isolation of the new isotope. When Meitner resumed work in 1917, though, not only Hahn but most of the students, laboratory assistants and technicians had been called up, so Meitner had to do everything herself. In February, Meitner extracted 2 grams of silicon dioxide () from 21 grams of pitchblende. She set 1.5 grams aside and added a tantalum pentafluoride () carrier to the other 0.5 grams, which she dissolved in hydrogen fluoride (). She then boiled it in concentrated sulfuric acid (), precipitated what was believed to be element 91, and verified that it was an alpha emitter. Hahn came home on leave in April, and together they devised a series of indicator tests to eliminate other known alpha emitters. The only known ones with similar chemical behaviour were lead-210 (which decays to alpha emitter polonium-210) and thorium-230.
For this more pitchblende was required. Meitner went to Vienna, where she met with Stefan Meyer. The export of uranium from Austria was forbidden due to wartime restrictions, but Meyer was able to offer her a kilogram of uranium residue, pitchblende from which the uranium had been removed, which was actually better for her purpose. The indicator tests showed that the alpha activity was not due to these substances. All that now remained was to find evidence of actinium. For this more pitchblende was required, and this time Meyer was unable to assist, as the export was now prohibited. Meitner managed to obtain 100 g of "double residue"—pitchblende without uranium or radium—from Friedrich Oskar Giesel and began tests with 43 grams of it, but its composition was different, and at first her tests did not work. With Giesel's help, she was able to produce a pure product that was strongly radioactive. By December 1917 she was able to isolate both the mother isotope and its actinium daughter product. She submitted their findings for publication in March 1918.
Although Fajans and Göhring had been the first to discover the element, custom required that an element was represented by its longest-lived and most abundant isotope, and brevium did not seem appropriate. Fajans agreed to Meitner naming the element "protoactinium" (subsequently shortened to protactinium), and assigning it the chemical symbol Pa. In June 1918, Soddy and John Cranston announced that they had independently extracted a sample of the isotope, but unlike Meitner they were unable to describe its characteristics. They acknowledged Meitner's priority, and agreed to the name. The connection to uranium remained a mystery, as neither of the known isotopes of uranium decayed into protactinium. It remained unsolved until the mother isotope, uranium-235, was discovered in 1929.