XYZ Line The XYZ Line , or Matallana Line , was a system of fortifications built during the Spanish Civil War in order to defend the capital of the Second Spanish Republic in Valencia , Spain . Unlike the Maginot Line , or the Iron Belt ( " " Cinturón de Hierro " " ) of Bilbao , which were expensive systems of fortifications and bunkers made of reinforced concrete , the XYZ Line was a simpler system of defenses in depth consisting of trenches and bunkers that took advantage of some of the most difficult terrain in Spain to the north and northeast of Valencia . The defenses were built to withstand bombardment by either heavy artillery or 1 , 000 - pound aerial bombs . Background . Following the success of the Aragon Offensive resulting in the Nationalist armies reaching the Mediterranean Sea , the Republic was split in two ; the Spanish Republican Army was in disarray and the road to Barcelona was open for conquest by the Nationalists . Even General Vicente Rojo said that Barcelona could have been taken with " less force and in less time " than in January 1939 . The Nationalist generals and Francisco Franco ' s German and Italian allies expected a swift attack against Barcelona . Nevertheless , Franco decided to turn south against the capital of the Spanish Republic at Valencia , because he feared French intervention in Catalonia , following the Anschluss . Furthermore , he didn ' t want a swift end to the war , as he wanted a war of annihilation against the Republic in order to crush all opposition . Dionisio Ridruejo said that : " A long war meant total victory . Franco chose the crueller option which , from his point of view , was also more effective . " Nationalist offensive . The Nationalist offensive started on 25 April , with General José Enrique Varela ' s Army Corps of Castille , Antonio Aranda ' s Galician corps , and Garcia Valiño ' s formation , but the advance was halted on 27 April . On 1 May , the Nationalists continued their offensive , advancing on three fronts from Teruel ( Varela ) , the Mediterranean sea coast ( Aranda ) , and a central column moving between them through the mountains ( Garcia Valiño ) . The Nationalists found it slow going due to rainy weather in March and April that slowed the offensive , the difficult terrain that aided the defense of Republican forces being driven back before them , and the dogged determination of the Republican troops , reinforced with new weapons brought in from France : Soviet " Supermosca " ( I - 16 Type 10 ) fighters with four machine - guns , 40 Grumman FF fighters and anti - aircraft guns . On June 13 , Castellón fell to Garcia Valiño ' s corps after several days of fighting but they were halted short of Sagunto where the mountains of the Sierra de Espadán came close to the sea . With the fall of Castellón , the Nationalists had a Mediterranean harbor into which munitions and food could be brought to the Nationalists troops in this front . The Nationalists were surprised by the resistance of the Republican forces , and General Alfredo Kindelán tried to persuade Franco to abandon the operation . The Condor Legion was exhausted and was eventually withdrawn from the front . Nevertheless , Franco ordered the attack to be continued and Valencia be captured by July 25 . By the beginning of July , the Nationalists reinforced the front with three divisions of the Corpo Truppe Volontarie ( CTV ) , led by General Mario Berti , and four divisions of General José Solchaga ' s Turia Corps . Furthermore , the Nationalists had nine hundred cannon and four hundred aircraft in this front , and had just received about 50 Italian medium bombers ( BR . 20 , SM . 79 , SM . 81 ) . Opposing the Nationalists , the Republicans had the six army corps ( Palacio ' s XVI Corps , Garcia Vallejo ' s XVII , Vidal ' s XIX , Duran ' s XX and Ibarrola ' s XXII , as well as Güemes ' s Group A and Romero ' s Group B ) of General Leopoldo Menéndez ' s Army of the Levante . On 5 July , Garcia Valiño attacked from Castellón , but he was stopped by the Republican forces led by Colonel Duran and General Menendez at the Sierra de Espadan . The final push of the Nationalist offensive began on 13 July . To the west , Solchaga ' s corps moved south from Teruel with Varela ' s corps and the Italian CTV on their flank to the east , and the Garcia Valiño formation tried to advance down the coast . After some determined resistance at Mora de Rubielos and Sarrion , the Republican line in the Sierra de Toro crumbled . The Nationalists then advanced 60 miles along a front of twenty miles wide until they were stopped by the XYZ Line fortifications running east and west from Viver into the Sierra de Espadan . The battle . Between July 18 and July 23 , these defenses , held by two Republican corps led by Colonels Ernesto Güemes and Carlos Romero , were successful in halting the Nationalist advance . The Nationalists tried to break the republican defenses with waves of infantry and intense bombing raids , but the Republican defenders through the use of well planned trenches and protected lines of communication were able to inflict heavy casualties on the Nationalists ( about 20 , 000 casualties ) , suffering relatively few themselves ( only 5 , 000 casualties ) . On July 23 , the offensive was halting and the Republican offensive in the north on July 24 , the Battle of the Ebro , prevented any further assaults on the line by drawing away eight Nationalist divisions and their heavy artillery . The Nationalist offensive ended less than forty kilometres from Valencia . Aftermath . According to Beevor , the defense of the XYZ Line was a far greater victory for the Republic than the Battle of Guadalajara . The Nationalists occupied the province of Castellon , but failed to occupy Valencia and suffered heavy casualties . The Republican Army had time to reorganize , and to plan the assault across the River Ebro . Furthermore , the Republican forces in Catalonia had time to rearm with the weapons received across the French frontier , reopened in March .