After Rey d'Harcourt's surrender, the Nationalist buildup began to tell on the Republican forces. With the weather clearing, the Nationalists started a new advance on 17 January 1938. Two days later, the Republican leadership finally gave up its scruples about the Battle of Teruel being an all-Spanish operation and ordered the International Brigades to join the struggle. Many of the units had been in the area but in reserve. Celebrities and politicians meanwhile entertained and visited the units. The US communist-sympathising singer Paul Robeson sang for them on Christmas Eve with a repertoire that included "L'Internationale" and ended with "Ol' Man River". Future British Prime Minister Clement Attlee, the left-wing Labour politician Ellen Wilkinson and future Labour government official and diplomat Philip Noel-Baker visited a British unit.