The war, inevitably, struck the Bilbao Club with force. It was not a time for leisure activities. The military mobilization affected Athletic players, in San Mamés funds were collected to fight the insurgent army, like that "pro-aviación Euzkadi" match of 1937; those are the years of the European and American tour by the Euzkadi national team... Of the 3000 members the club had before the civil war, they dropped to 587; in a country where phrases such as "ration cards", "evacuation" and "exile" were minted, it was difficult to reorganize the team. However, little by little, "the return to normality" prevailed.
Perhaps, one of the greater accomplishments in Athletic's history has been to successfully achieve the complete renovation of the team after the war. The great champion team prior to the conflict was dismantled after the war, and it became necessary to create another that would be able to attain the same sporting accomplishments as its predecessors. Only Gorostiza, Oceja, Unamuno, Garate and Urra were still donning the red-white jersey. The system for recruiting new players was the organization of a tournament among teams from Biscay that had at least five members younger than 17 years of age. The wealth of the Biscayan cantera (youth teams) was soon validated, seeing that from this competition, figures such as Bertol, Panizo, Arqueta, Gainza, which Zarra or Iriondo who were to be included shortly, arose. This spot on policy of signings lead to Athletic being able to form one of the most successful and remembered teams in its history. Similarly the reappearance of the team made the members return, as there were 1500 members registered for the 1940-41 season.
The promises soon became real, and the postwar period team won nothing less than 5 titles (the 1942-43 League and the 43, 44, 45 and 50 Copas), as well as being runners-up on 6 occasions (3 Leagues and 3 Copas). As a result Athletic got back on track, connected with its supporters, as shown by the spectacular welcomings with which they were received, and were considered the Copa Kings having made it to 7 finals in 13 years. In the words of famous journalist Monchin, "the final will be played between Athletic and another team".
This is the time of the great rojiblanco forward line (Iriondo, Venancio, Panizo, Zarra and Gainza), although it would be unfair to forget the rest of the magnificent team: Lezama, Oceja, Bertol, Ortuzar, Mieza, Arqueta, Ortiz, Garate, Celaya, Unamuno... It was therefore a great team where teamwork was prioritized but in which the great quality of some individualities also stood out: Zarra, the best goalscorer in the history of the League, 6 times Pichichi -top goalscorer- and authentic social phenomenon, Panizo the player who changed the conception of football in Bilbao, Gainza the fireproof winger...
On the other hand, the competitions were becoming more and more demanding. The League went from twelve teams in the 1939-40 season to fourteen in 1942-43 and sixteen in the 1951-52 season. Also, the first foreign players (Kubala, Ben Barek, Di Stéfano...) began to arrive.
During these years the most radical and definite transformation was made in San Mamés with the construction of a new storey in the main grandstand which was now without any columns and with just one icon which would become the symbol of the field as of the 1952-1953 season: the 'san Mamés Arc".
