<jats:title>Abstract</jats:title>
<jats:p>While World War I rages on, life in Leiden continues more or less as usual. Ehrenfest begins to develop what will become his ‘adiabatic hypothesis’. With Lorentz, he also tries—often in vain—to save what is left of the international relations between European scientists. In the summer of 1915, Afanassjewa gives birth to a son, Pawlik.</jats:p>