Complex sentence: In 1936, Latvia, keen to replace its elderly Bulldogs with more modern aircraft, sold 11 Bulldogs to Basque nationalist forces; becoming part of the Spanish Republican Air Force in the Spanish Civil War, they remained in use until the Battle of Santander.
Split into simpler sentences: In 1936, Latvia, intent on replacing its Bulldogs with more modern aircraft, sold 11 Bulldogs to Basque nationalist forces. These became part of the Spanish Republican Air Force in the Spanish Civil War; remaining in use until the Battle of Santander.