At this time, the Hungarian nation numbered around 400,000 people. Early modern period The first accurate measurements of the population of the Kingdom of Hungary including ethnic composition were carried out in 1850–51. There is a debate among Hungarian and non-Hungarian (especially Slovak and Romanian) historians about the possible changes in the ethnic structure of the region throughout history. Some historians support the theory that the proportion of Hungarians in the Carpathian Basin was at an almost constant 80% during the Middle Ages. Non-Hungarians numbered hardly more than 20% to 25% of the total population. The Hungarian population began to decrease only at the time of the Ottoman conquest, reaching as low as around 39% by the end of the 18th century. The decline of the Hungarians was due to the constant wars, Ottoman raids, famines, and plagues during the 150 years of Ottoman rule. The main zones of war were the territories inhabited by the Hungarians, so the death toll depleted them at a much higher rate than among other nationalities. In the 18th century, their proportion declined further because of the influx of new settlers from Europe, especially Slovaks, Serbs and Germans. In 1715 (after the Ottoman occupation), the Southern Great Plain was nearly uninhabited but now has 1.3 million inhabitants, nearly all of them Hungarians. As a consequence, having also the Habsburg colonization policies, the country underwent a great change in ethnic composition as its population more than tripled to 8 million between 1720 and 1787, while only 39% of its people were Hungarians, who lived primarily in the centre of the country. Other historians, particularly Slovaks and Romanians, argue that the drastic change in the ethnic structure hypothesized by Hungarian historians in fact did not occur. In particular, there is a fierce debate among Hungarians and Romanian historians about the ethnic composition of Transylvania through these times. For instance, Ioan-Aurel Pop argues that the Hungarian army of 9-10th centuries, while it was eminently suitable for raids, was not at all fit to occupy territories already densely inhabited, especially in the hilly and mountainous areas. He adds that Hungarians, outside of Alföld, region where they were seminomadic during this time, were not able to become colonizers, and that for this reason the regions of Transylvania, Upper Hungary and Croatia were integrated in the Hungarian Kingdom in a later stage, after the year 1000, after the sedentarization, Christianization