Seven more-or-less well documentated cases of the use made of icebound sea areas in winter for the purposes of warfare are reviewed. The sea-ice crossings took place in 1495, 1577, 1581, 1658, 1809, 1940, and 1943, i.e. the first five occurred during the Little Ice Age. A book authored by a prominent Swedish personality (Archbishop Olaus Magnus) and published in 1555 says that warfare on frozen sea areas in winter by the Muscovites (Russians) and Swedes was as common as warfare by ships on the open seas in summer. There are indications of some crossings of ice-bound seas prior to 1495 and not necessarily for warlike activities. It seems that the Vikings too did some sea-ice crossings.
The crossings of 1495, 1577, 1581, and 1940 involved the Gulf of Finland, that of 1809 the Gulf of Bothnia and the Aaland Islands area of the Baltic, that of 1658 the Danish Belts, and that of 1943 the Gulf of Taganrog in the Sea of Azov. In the first three cases the powers concerned were Muscovy (Russia) and Sweden which for centuries were fighting for supremacy in the Baltic and over the routes from the inner Baltic (Gulf of Finland and Bay of Riga) to western Europe. The case of 1809 involved, again, Russia and Sweden, though in the background of the conflict between the two were wider European issues of the Napoleonic wars. The 1658 crossing of the frozen-over Danish Belts was accomplished by the Swedes, forcing the Danes into submission: In the ensuing Peace Treaty Sweden for the first time in her history achieved her present territorial extent in the Scandinavian Peninsula. The case of 1940 was connected with the 1939–40 Winter War of Soviet Russia against Finland. The crossing of 1943 was made by German forces retreating from the Caucasus under the pressure of Soviet forces in World War II.
The crossings of 1577, 1581, 1658, 1809, 1940, and 1943 took place between late in January and late in March; the case of 1495 appears to have taken place early in the winter season: probably late in November. Since in the period 1931–60 no part of the Gulf of Finland froze over before about the middle of December, the early date of the crossing of 1495 is possibly one of the many indications of cold winters during the Little Ice Age.