The Japanese 31st Division's thrust at Kohima had been a costly failure, eventually forcing it to a disastrous retreat. The Japanese 33rd Division, Japanese 15th Division and the INA 1st Division had suffered a similar fate at Imphal. The Japanese and their allied forces lost at least fifty thousand dead. In previous years there used to be lull during the monsoon period but not this year (1944–45). The command decided that it was time to target the heart of Japanese Army in Burma. General William "Bill" Slim's plan through the monsoon months had been pursuit of the defeated Japanese in Kohima and plains of Imphal with 5th Indian Infantry Division down the Tiddim Road and 11th East African Division down the Kabaw Valley, until the two joined at Kalemyo. The next offensive plan centred on the occupation of Central Burma, as far south as Mandalay to exploit further south and destroy the Japanese forces in the Shewbo Plains, north of Irrawaddy, where armour could be used. Allied forces crossed River Chindwin and the spearheads of both corps of the Fourteenth Army (IV and XXXIII Corps) were moving into selected battle areas, which surprised the Japanese as they had not anticipated any major operations during the monsoon period. The British Fourteenth Army was now faced with a major obstacle (the Irrawaddy) covered by determined Japanese. The Irrawaddy River in its middle is about broad and dotted with treacherous and shifting sand bars. An opposed direct crossing would have been very expensive with low percentage of success.