After the outbreak of the Spanish Civil War, he left for Spain, arriving there in October 1936, during the Battle for Madrid. He joined the Romanian Motorized Artillery Regiment of the XI International Brigade, commanded by Valter Roman. In 1937, he participated in the battles of Brunete, Quinto-Belchite and Teruel. In December 1937, during the latter confrontation, his leg was injured, but Cristea refused to be hospitalized, deciding instead to remain present on the battlefield. Afterwards, he was promoted to captain and became the commander of the "Tudor Vladimirescu" Battery. At the Battle of the Ebro, he had an important role in the crossing of the river. Demobilized along the other volunteers in December 1938, he continued to fight in Catalonia in January–February 1939.