Sentence:  Despite the successful rehabilitation of over a half million disabled persons in the first eleven years after 1943 , the existing program was still seen to be inadequate to cope with the nation 's <WSD>backlog</WSD> of an estimated two million disabled . 
Meaning of the word marked with <WSD> tags:  an accumulation of jobs not done or materials not processed that are yet to be dealt with (especially unfilled customer orders for products or services)