Many theories are posited for the cause of the Viking invasions; the will to explore likely played a major role. At the time, England, Wales, and Ireland were vulnerable to attack, being divided into many different warring kingdoms in a state of internal disarray, while the Franks were well defended. Overpopulation, especially near the Scandes, was possibly influential (this theory regarding overpopulation is disputed). Technological advances like the use of iron and a shortage of women due to selective female infanticide also likely had an impact. Tensions caused by Frankish expansion to the south of Scandinavia, and their subsequent attacks upon the Viking peoples, may have also played a role in Viking pillaging. Harald I of Norway ("Harald Fairhair") had united Norway around this time and displaced many peoples. As a result, these people sought for new bases to launch counter-raids against Harald.