War of the Two Peters The War of the Two Peters ( , ) was a war fought from 1356 to 1375 between the kingdoms of Castile and Aragon . Its name refers to the two rulers of these countries : Peter of Castile and Peter IV of Aragon , respectively . One historian has written that " all of the centuries - old lessons of border fighting were used as two evenly matched opponents dueled across frontiers that could change hands with lightning speed . " Background . At the beginning of the fourteenth century , Castile was suffering from unrest caused by its civil war , which was fought between the local and allied forces of the reigning king , Peter of Castile , and his half - brother Henry of Trastámara over the right to the crown . Casus belli . Peter IV of Aragon supported Henry of Trastámara . Henry was , in turn , supported by the French in the form of Bertrand du Guesclin and his " free companies " of troops . Peter of Castile was supported by the English . The War of the Two Peters can thus be considered an extension of the wider Hundred Years ' War as well as the Castilian Civil War . The Castilian king ' s aims were to lay claim to the Kingdom of Valencia , which included parts of Murcia , Elche , Alicante and Orihuela . Peter of Aragon wished to dominate the Mediterranean in opposition to Castile and Castile ' s ally Genoa . A naval incident between the two powers had already caused tension : Catalan galley s , armed by Mossèn Francesc de Perellós , who had letters of marque from the Aragonese king , aided France against England , and also managed to capture two Genoese ships at Sanlúcar de Barrameda . Genoa was an ally of Castile . Peter of Castile , leading the Castilian fleet , caught up to Perellós at Tavira , but was unable to capture him . War 1356 – 1363 . The war lasted from 1356 to 1375 . Prolonged because Peter of Castile ended up losing his throne to Henry of Trastámara , the war primarily took place on the border between Castile and Aragon , namely Aragonese border towns such as Teruel , which fell to the Castilians . In 1357 , Castile penetrated Aragon and conquered Tarazona on March 9 . On May 8 , they arranged a temporary truce . At the beginning of 1361 , the Castilians conquered the fortresses of Verdejo , Torrijos , Alhama , and other places . However , the " peace of Terrer " ( sometimes called the " peace of Deza " ) was negotiated on May 18 , 1361 , in which all conquered places and castles were returned to their original lords . Bernardo de Cabrera , ambassador of the Aragonese king , negotiated the peace . Peter IV married his daughter Constance to Frederick III the Simple . In June 1362 , Peter of Castile met with Charles II of Navarre at Soria , in which mutual aid was promised . He also contracted an alliance with Edward III of England and Edward ' s son The Black Prince . With these negotiations complete , the Castilian king invaded Aragonese territory without officially declaring war , and the conflict commenced again . The Aragonese king was at Perpignan without troops , and thus caught off guard . The Castilians took the castles of Arize , Atece , Terrer , Moros , Cetina and Alhama . Peter of Castile was unable to take Calatayud , even though he attacked it with all types of siege machines . Without taking his conquests any further , he returned to Seville . The next year ( 1363 ) Castile continued the war against Aragon , and again occupied Tarazona . Peter of Castile received reinforcements from Portugal and Navarre . Meanwhile , the Aragonese king negotiated a treaty with France and a secret treaty with Henry II of Castile . Pedro of Castile then conquered Cariñena , Teruel , Segorbe , Morvedre , Almenara , Xiva , and Bunyol . The papal nuncio Jean de la Grange arranged the " peace of Morvedre " ( Sagunt ) ( July 2 , 1363 ) between the two kings . However , it was not ratified , and hostilities continued . The Castilians penetrated the Kingdom of Valencia in 1363 , and conquered Alicante , Caudete , Elda , Gandia , and other places . War 1363 – 1369 . From 1365 to 1369 Peter of Castile was preoccupied with maintaining his position on the Castilian throne against Henry of Trastámara . In 1366 began the Castilian Civil War which would see Peter of Castile dethroned . He was assailed by his illegitimate brother Henry of Trastamara at the head of a host of soldiers of fortune , including Bertrand du Guesclin and Hugh Calveley , and abandoned the kingdom without daring to give battle , after retreating several times ( first from Burgos , then from Toledo , and lastly from Seville ) in the face of the oncoming armies . Peter fled , with his treasury , to Portugal , where he was coldly received by his uncle , King Peter I of Portugal , and thence to Galicia , in northern Spain , where he ordered the murder of Suero , the archbishop of Santiago , and the dean , Perálvarez . Peter of Castile was overthrown in 1369 . Valencia and Granada . The Kingdom of Granada supported Peter of Castile in this conflict . During the War of the Two Peters , southern Valencia saw a lot of low - level ravaging and political instability as Castilian troops and their Moorish allies invaded this territory . The Castilians unsuccessfully laid siege to Orihuela in 1364 . The war ' s end . This border conflict finally ended with the " peace of Almazán " ( 1375 ) leaving no clear victors . Castile recovered comarca s that had passed under Aragonese rule , such as the seignory of Molina . A marriage was contracted between Eleanor of Aragon , daughter of Peter IV of Aragon , and John I of Castile , the successor of Henry II of Castile . The misery of this war was worsened by the Black Death and other natural disasters , such as drought and a plague of locust s . The war ruined the Aragonese economy , coinciding with a decrease of the country ’ s population caused by the war and the Black Death . The cathedral of Tarazona was destroyed during the war , and not rebuilt until much later . However , the war is believed to have led to the establishment of administrative and military forces that would ultimately result in a unified Castile and Aragon in the next century .