Complex sentence: As a counter-measure the Republicans sent a detachment of Russian tanks led by General Pavlov and two International Brigades (XII and XIV) at Boadilla and re-occupied it, yet they finally became cut off in the town by Nationalist's counter-attacks.
Split into simpler sentences: As a counter-measure the Republicans sent a detachment of Russian tanks led by General Pavlov and two International Brigades (XII and XIV). The Republican XII International Brigade counter-attacked at Boadilla and occupied it, but they were cut off in the town by Nationalist's counter-attacks.