In Treue fest - Motto of the House of Wittelsbach; "steadfast in loyalty; firm in fidelity"
Having achieved the highest heights of its power yet with the election of Louis IV of Bavaria as Holy Roman Emperor in 1328, the Wittelsbachs during his long life, wherein he was King of the Romans as early as 1314, would gradually expand into holding various territories across the Empire; namely a unified Duchy of Bavaria, the Electorate of the Palatine, the Margravate of Brandenburg, the Counties of Holland, Hainaut, Zeeland, and Tyrol.
Much of this, in fact nearly all of it, had been achieved at the expense of various German Princes, and nobles--especially those of the House of Luxembourg, who would even elect an Anti-King in the form of the future Emperor Charles V in the last years of Louis IV's life.
What had turned out to be an unexpected boon to the Emperor however, as sad and painful as it seemed at the time, would be the death of much of his family during the Black Death, thereafter only having as heirs his eldest, Louis of Brandenburg, and youngest Otto; this state of affairs allowing, following Louis IV's death in 1347, the avoidance of a ruinous division of the Wittlesbach lands.
Louis of Brandenburg would inherit Bavaria, Brandenburg and Tyrol, while Otto would inherit the Low Country lands of Holland, Hainaut, and Zeeland; effectively splitting the Wittelsbachs into a total of three branches; an elder Bavarian branch, the Palatinate branch held by the nephews of Louis IV and their descendants, and the Low-Countries branch.
The dynasty though, due to excommunications, and other issues of that kind, would be robbed of having the Imperial Crown a second time; with it passing to their long-time rival Charles V of Luxembourg, who would shortly thereafter begin a long feud that dragged on; attempting to wrestle Brandenburg directly from Louis of Brandenburg.
Charles would have early successes, but by 1350 he was forced to accept the investment of the Wittelsbachs as Margraves of Brandenburg. This had been achieved through a combination of Danish assistance, and the aid of the Habsburgs, who were by now loyal friends of the Wittelsbachs, under Rudolf the Founder. In order to gain the support of Rudolf, Louis had given up Tyrol to them, and thus from there Charles V would find himself pressured on two fronts.
Come the Golden Bull of 1356 the Wittelsbachs were able to force through the evolution of Bavaria, considered an ancient 'Stem Duchy' of Germany, to the rank of Electorate; at the cost of Brandenburg losing this status. To punish this, Charles V would ensure their Habsburg allies themselves would not get an Electorate, resulting in the forging of the Privilegium Maius by Rudolf, as well as the cooling of relations between the two former allies.
The rise of Bavaria to Electorate, and the privileges of Primogeniture that it gave, would firmly tie the possessions of the Bavarian branch together within Bavaria itself; giving them a strong power base.
By 1370 the Bavarian branch, ruled by Meinhard III Wittelsbach, is one of the key realms within the Empire; rivalled only by the growing power of the Habsburgs, and their rivals the Luxembourgers.
