The Little Entente comprised Czechoslovakia, Yugoslavia and Romania, and large chunks of the first two nations had been carved out of the Austrian Empire. Their inhabitants had never been fans of the Austrian rule and regardless of actual personalities, the Habsburgs appeared in a negative light to many of their former subjects, of the Kingdom of Bohemia or states such as Bosnia Herzegovina, for example. France, no friend to the Habsburgs, had backed the formation of the Little Entente, which was intent on preserving its individual nations' sovereign status and opposing any hint of Habsburg monarchical ambitions. Karl had a hereditary claim to the Kingdom of Bohemia and former states under the Austrian Empire and the LE could well have seen in a new Hungarian Habsburg monarch a nation's excuse for doing a bit of land-grabbing from its neighbours - whether or not Karl wanted it. WWI had seen many rulers fail to check national ambitions - no one thought of the Emperors Franz Josef or Nicholas II as being rabid warmongers for example, but they were propelled to war nevertheless - and Karl's political potential may well have appeared far too threatening to the LE and its backers despite his obvious personal lack of desire for conquest. So really, Karl's personality had nothing to do with the fears and aims of the Little Entente, which were basically focused on his imperial name. But his failure to grasp the realpolitik of this very complex and volatile situation would have been another reason for Horthy to decline to back such a sadly naive Habsburg.
