Strange that a little fish should have had such great power over mankind; yet it is not going beyond the strict truth to state that the mysterious wanderings of the herring determined throughout several centuries the whole course of northern commerce. During the Middle Ages, upon the appearance of the herring now on this coast and now on that, the wealth and prosperity of the whole districts depended. Herring fishing became a branch of industry that decided the fate of nations. To it the Hansa owes a large portion of its riches and its power; in the herring fisheries, when in the year 1425 the fish began to spawn in the German Ocean, the Netherlands found the foundation stone of their wealth and dignity. Indeed, it was said later, with scant exaggeration, that Amsterdam was built upon herrings. Now, as masters of the Belt and the Sound, the Danes were able, if they chose, greatly to harass the Hanseatic traders and fishermen. For many years they had not put forth their power, or rather the Hanseatic towns, with the diplomatic astuteness that greatly distinguished them, had averted the possibility of such danger by wise concessions of tributes and privileges. Still disputes would arise, things did not always go off peaceably, and in 1227 the Hansa towns won their first military laurels, defeating the Danes in the battle of Bornhöved and permanently weakening the power of their troublesome neighbours in Northern Germany. A few years later Lübeck, almost unassisted, threatened in its independence by the Danish king, won a great naval victory over its neighbours; and gained yet another in 1249, when Eric II. had ventured to attack some of its ships upon the open seas. On this occasion the merchant townsmen even seized and sacked Copenhagen and planted their flag in Zealand. It was no very easy position which the Baltic cities (for it was they who were chiefly threatened) had to maintain against the Danish kings as the power of the latter increased. For with their power, their rapacity and cupidity increased also, and this made them look on the rich commercial towns with a longing desire to absorb them into their own possessions. These, though extensive, were poor, and their inhabitants neither industrious nor prosperous. Further, the Danes, Norwegians, and Swedes were in constant feud with one another, and each of these states turned an eye of greed towards the flourishing Baltic cities, whose possession they coveted. The two Scandinavian powers, in particular, constantly harassed the German merchants by their scanty comprehension of treaty rights, their breaches of faith, and it was not easy work for the cities to steer clear between the three kingdoms, that were now at deadly feud with one another, now convulsed by civil wars, now united in a policy of rapine.