The Second Crusade, the second major crusade launched from Europe, is called in 1145 in response to the fall of the County of Edessa the previous year.

Edessa is the first of the Crusader states to have been founded during the First Crusade (1095–1099), and is the first to fall.

The Second Crusade is announced by Pope Eugene III, and is the first of the crusades to be led by European kings, namely Louis VII of France and Conrad III of Germany, with help from a number of other important European nobles.

The armies of the two kings march separately across Europe and are somewhat hindered by Byzantine emperor Manuel I Comnenus; after crossing Byzantine territory into Anatolia, both armies are separately defeated by the Seljuk Turks.

Louis and Conrad and the remnants of their armies reach Jerusalem and, in 1148, participate in an ill-advised attack on Damascus.

The crusade in the east, a failure for the crusaders and a great victory for the Muslims, will ultimately lead to the fall of Jerusalem and the Third Crusade at the end of the 12th century.The only success comes outside of the Mediterranean, where Flemish, Frisian, Norman, English, Scottish, and some German crusaders, on the way by ship to the Holy Land, fortuitously stop and help the Portuguese in the capture of Lisbon in 1147.

Some of them, who had departed earlier, have helped capture Santarém earlier in the same year.

Later they also help to conquer Sintra, Almada, Palmela and Setúbal, and are allowed to stay in the conquered lands, where they will produce offspring.

Meanwhile, in Eastern Europe, the first of the Northern Crusades begin with the intent of forcibly converting pagan tribes to Christianity, and these crusades are to continue for centuries.