At their best, the various forms of Protestantism had never won more than a scanty noble and intellectual elite of the nation; they had never taken root among the peasantry or the petty bourgeoisie. Whilst the gradual effacement of reformed creeds removed a powerfully creative intellectual and literary factor from Poland's life, the re-establishment of Catholicism restored to the republic that spiritual unity which was to be the chief source of national strength in the coming struggle against the aggression both of Orthodox Russia and of Lutheran Germany.
Access to the Baltic had been a vital question since the dawn of the Polish State in the loth and i i th centuries. Poland's expansion eastward, which began definitely in the 14th century through the acquisition of Red Russia by Casimir the Great, and was continued in the 15th through the dynastic union with Lithuania, made an exten sion of her foothold on the Baltic shore imperative. At the same time, the knights of the Teutonic Order (q.v.) had threatened to cut Poland off from access to the sea altogether. They had been beaten down by the earlier Jagiellons, and access to the sea secured by way of Danzig. But in the 16th century, the foe survived and began to regain strength in the secular and Protestant duchy of East Prussia, formally owning allegiance to Poland. Sigismund II. was naturally attracted by an opportunity to outflank this foe and to gain a separate outlet to the sea.
In the middle of the 16th century the ancient order of the Knights of the Sword, whose territory embraced Estonia, Livonia, Courland, Semgallen and the islands of Dag6 and Oesel, was tottering to its fall. All the Baltic powers were more or less interested in the apportionment of this vast tract of land, whose geographical position made it not only the chief commercial link between east and west, but also the emporium whence the English, Dutch, Swedes, Danes and Germans obtained their corn, timber and most of the raw products of Lithuania and Muscovy. Poland and Muscovy as the nearest neighbours of this moribund state, which had so long excluded them from the sea, were vitally con cerned in its fate. After an an archic period of suspense, lasting from 1546 to 1561, during which Sweden secured Estonia, while Ivan the Terrible fearlessly rav aged Livonia, Sigismund II., to whom both the grand-master of the Order and the archbishop of Riga had appealed more than once for protection, at length in tervened decisively. At his camp before Riga in 1561, the last grand-master, who had long been at the head of the Polish party in Livonia, and who had embraced Protestantism, and the arch bishop of Riga, gladly placed themselves beneath his protection, and by a subsequent convention signed at Vilna (Nov. 28, 1561),
Livonia was incorporated with Lithuania in much the same way as Prussia had been incorporated with Poland 36 years previously, that is to say, as a new Protestant duchy, and as a fief of the Polish Crown, with local autonomy and freedom of worship.
The danger to Lithuania, re vealed in the Baltic wars with Ivan the Terrible, as well as the apathy shown in these matters by the diet of Poland, must have convinced so statesmanlike a prince as Sigismund II. of the neces sity of preventing any possibility of cleavage in the future between the two halves of his dominions. A personal union under one monarch had proved inadequate. A further step must be taken— the two independent countries must be transformed into a single state. The principal obstacle was the opposition of the Lithuanian magnates, who feared to lose their dominance in the grand-duchy if they were merged in the szlachta (gentry) of the kingdom. When things came to a deadlock in 1564, the king tactfully inter vened and voluntarily relinquished his hereditary title to Lithu ania, thus placing the two countries on a constitutional equality and preparing the way for fresh negotiations. The death, in 1565, of Black Radziwill, the chief opponent of the union, still further weakened the Lithuanians, but the negotiations, reopened at the diet of Lublin in 1569, at first also led only to rupture. Then Sigismund executed his master stroke. Knowing the sensitiveness of the Lithuanians as regards Volhynia and Podolia, he suddenly, of his own authority, formally incorporated both these provinces with the kingdom of Poland, whereupon, amidst great enthusiasm, the Volhynian and Pedolian deputies took their places on the same benches as their Polish brethren. The hands of the Lithuani ans were forced. Even a complete union on equal terms was better than mutilated independence. Accordingly they returned to the diet, and the union was unanimously adopted on July 1, 1569. Henceforth the kingdom of Poland and the grand-duchy of Lithuania were to be one inseparable and indivisible body politic ; all dependencies and colonies, including Prussia and Livonia, were to belong to Poland and Lithuania in common. The retention of the old duality of dignities was the one reminiscence of the original separation—and was only abolished in 1791, four years before the final partition of Poland.