They were an illegitimate cadet line of the House of Burgundy who acceded to power in Castile in 1369 as a result of the victory of Henry of Trastámara over his half-brother Peter I in the 1351–1369 Castilian Civil War, in which the nobility, and, to a lesser extent, the clergy had played a decisive role in favour of the former.

The dynasty was replaced by the House of Habsburg upon the effective enthronement of Charles V as king of Castile and Aragon in 1516, even though his mentally incompetent and imprisoned mother Joanna lived until 1555.

Gaining support throughout Castile because of his relation to Alfonso XI and Peter's continuous military escapades, Henry built an alliance with Aragon and France, including mercenaries led by French constable Bertram Du Guesclin for another attempt at the Castilian crown in 1365.

On 13 April 1367, Peter and Edward's forces strongly defeated the armies of Francs, Aragonese, and Castilians led by Henry and captured Bertram Du Guesclin.

Under Henry, a new nobility rose in prominence to gain land grants of large estates and vast royal privileges.

[3] This marriage by Henry's son would eventually put the Trastámaras in control of both Castile and Aragon, comprising a majority of the Iberian Peninsula.

On the basis of this marriage, John made an unsuccessful claim to the throne of Portugal upon Ferdinand I's death in 1383, a move that possibly could have led to the unification of all of the Iberian Peninsula.

He waited only two years to independently take control of the throne in 1393 at only fourteen, amidst a great deal of violence being carried out against Jews throughout Castile.

During his time as regent, Ferdinand was chosen as the ruler of Aragon, due to his maternal relation to the Aragonese throne through the Compromise of Caspe in 1412.

Now John II's constable, Don Alvaro, agreed to a basically victorious truce, as the Aragonese branch of Trastámaras was removed from Castile.

[6] John II's authority continued to decline following this military engagement, and he eventually ceded all power to Don Alvaro, who created an oligarchy of nobles.

Henry IV of Castile was an unpopular ruler, in part because of his taste for Moorish fashion and his disagreement with military engagement with Granada.

Amidst the struggle to settle the ensuing claims to the throne, Henry's wife Joan became pregnant again while being held as a hostage of a noble family.

This sign of misbehavior further weakened her daughter Princess Joan's claim to the throne, and paved the way for Henry's half-sister Isabella to take power.