Sleswick (Danish Slesvig; Ger. Schleswig) forms, united with the former duchies of Holstein and Lauenburg, a province of Prussia, having the Baltic Sea and the territories of Lübeck and Mecklenburg for its eastern boundary, Hamburg and Hanover for its southern, the North Sea for its western, and Jutland (Denmark) for its northern. Area, 7273 sq. m. It belongs to the alluvial peninsula of Jutland, its eastern half being an undulating plain and its western a series of low-lying but very fertile marsh-lands, protected from the ocean by sea-banks. The eastern coast (230 miles) is deeply indented by several long narrow arms of the sea called fjords, some of which make excellent harbours, and alongside it lie the large islands of Fehmern and Alsen. The western coast is more regular, but very low; and a string of low islands (Sylt, Föhr, Pellworm, Nordstrand, and others) and sandbanks stretching right in front of it protect it to some extent from the waves of the North Sea. The Elbe (on the southern border) and the Eider are the most important rivers; but the province is crossed by the North Sea and Baltic Canal (61 miles long), the Eider Canal (20), the Stecknitz Canal (35), and two or three shorter ones. The chief occupations of the people are agriculture, cattle-feeding, shipbuilding, and navigation. Corn and fat cattle are exported, the latter (of an excellent breed) in large numbers to England. Nearly 87 per cent. of the total area is under cultivation. Fishing is carried on in the inlets of the Baltic, and there are profitable oyster-beds amongst the sandbanks of the North Sea. Flensburg is the chief seaport. Pop. (1890) 1,217,393—i.e. for Sleswick and Holstein combined. Of this number nearly all belong to the Low German stock (except 140,000 Danes in the north), and are Protestants in religion. Sleswick-Holstein sends 10 members to the imperial parliament, and 19 to the Prussian house of representatives, and has an assembly of its own consisting of 58 members.
At the dawn of history Sleswick was inhabited by the Cimbri; they were succeeded by the Angles, Jutes, and Frisians. But the greater part of the Angles crossed over into England, and their place was taken by Danes. When Charlemagne reduced the Saxons to his sway the Danish king built a wall from sea to sea alongside the Eider, the southern frontier of his domains, to protect himself against the emperor's attacks. And from that time (808) for 350 years Sleswick alternated between the Danish and the imperial yoke, its duke paying homage sometimes to the king of the Danes, sometimes to the German emperor. In 1157 the duchy was definitively united with the Danish kingdom. Meanwhile Holstein to the south was conquered and christianised by Charlemagne and his successors, and the county of Holstein formed in 934 by the German king Henry I. Between 1157 and 1225 this southern part of the peninsula was subject to the rule of Denmark. From the year 1386 the Danish duchy of Sleswick and the German county of Holstein had one common ruler, the Count of Holstein having fallen heir to the former dignity in that year. About fifty years earlier a compact had been made, the Constitutio Waldemariana, by which it was solemnly agreed that the crown of Denmark and the duchy of Sleswick should not in future be held by the same individual ruler. In spite of this agreement, however, King Christian I. of Denmark possessed himself of both Sleswick and Holstein, and, having in 1474 acquired from the Emperor Frederick III. the suzerainty of Ditmarsh, he converted the county of Holstein into a duchy. The sons of the next Danish king divided his territories amongst them after his decease, and their immediate successors still further subdivided them; but eventually they were all gathered up again (1581) under the heads of the royal line (Glückstadt) and the ducal line (Gottorp). The latter held Sleswick as a fief of Denmark and Holstein as a fief of the German empire; nevertheless the king of Denmark ruled in several detached portions of both duchies.
Ten years after the conclusion of the Thirty Years' War the Duke of Sleswick (Frederick III.), backed by his son-in-law Charles X. of Sweden, declared himself an independent and sovereign prince. This the Danish kings all along resented, and at length, when Charles XII. of Sweden was routed at Pultowa, Frederick IV. of Denmark grasped (1721) the opportunity, invaded Sleswick, expelled the duke, and seized his territories in that duchy. The heads of the ducal line became, one in 1751 king of Sweden, the other (the nominal duke) in 1762 emperor (Paul III.) of Russia, and soon ceased to take any interest in Sleswick-Holstein, the whole of which (both duchies) was formally given up to the king of Denmark in 1767. On the dissolution of the German empire in 1806 Holstein was united with the Danish crown, but was reincorporated in the German Confederation by the Congress of Vienna in 1815. From 1767 the Danish sovereigns had steadily endeavoured to make the duchies thoroughly Danish and to reduce them to complete dependence upon the Danish crown. At length in 1846 King Christian VIII. proclaimed that the law of succession in Sleswick should be the same as for the Danish crown—in other words, that Sleswick was to be made an integral part of the Danish kingdom. This action, as well as the refusal of the king to summon the common estates of the joint duchies, was warmly and actively resented by the people of Sleswick and by the German party in both Sleswick and Holstein. At length (March 1848) the latter, headed by Count Reventlow and Prince Frederick of Augustenburg, rose in revolt. They found a zealous supporter in Frederick William IV. of Prussia, who made himself the executive agent of the German Confederation. War began in March 1848; several severely contested battles, as at Düppel, were fought before peace was made in the end of 1850. Through the intervention of Austria matters were for a time put on a footing satisfactory to the people of the two duchies—so far at least as fair words and fine promises could go. The Danes, however, still pursued their policy of forcible denationalisation in both Holstein and Sleswick. The death of King Frederick VII. of Denmark in the end of 1863 brought the question of succession to a crisis; for Frederick of Augustenburg pro- claimed himself Duke of Sleswick, to which title the new king of Denmark likewise laid claim. Austria and Prussia called upon Duke Frederick to abdicate and leave the duchy, and forbade Denmark to proclaim a constitution for it. Both refused to obey. Thereupon the two powers took up arms against the northern kingdom, and after a short but decisive campaign wrested both Holstein and Sleswick from the Danes. How they divided their conquest between them, and then quarrelled over the division, and how Austria came out of the conflict thoroughly worsted, has been already told under Germany (q.v., p. 183). Since the conclusion of that war (1866) Sleswick-Holstein has constituted a province of the kingdom of Prussia. In the northern part of Sleswick, mainly Danish in population, German measures, such as the compulsory use of the German language in schools, &c., have been considered oppressive; and so recently as 1898 the German government was expelling recalcitrant Danes across the frontier.
Sleswick, the capital of Sleswick-Holstein, stands on the Schlei inlet of the Baltic, 28 miles NW. of Kiel. It was a great trading town in the 9th century, and has a Gothic cathedral and the Gottorp Castle. Pop. (1885) 15,187; (1895) 17,255.