This paper explores the history of two islands of the Danube River, their movements due to the hydrodynamics of the river and the geopolitical consequences of these movements. The paper shows the process through which the Danube with its hydrological dynamics became a historical actor that provoked military actions, shifted political borders and was the source of intense diplomatic debates between Romania and Bulgaria at the end of the nineteenth century and the beginning of the twentieth. The paper attempts to contribute to a stream of theory generally known as posthumanism by bringing to the forefront historical facts from the nineteenth century and the first years of the twentieth. It also contributes to enriching the environmental history of Southeast Europe.