From
毛泽东 《七律·长征》 (October 1935), written at the end of the Long March:
红军不怕远征难,
万水千山只等闲 (The Red Army fears not the hardship of the long march; ten thousand waters, thousand mountains - mere trifles).
大渡桥 refers to the chain bridge at
泸定 (Luding) over the Dadu River, stormed by 22 Red Army soldiers on 29 May 1935 - the iconic Long March crossing.