Has a national football team ever declined to participate in the World Cup finals after qualifying?
If there has been such a case, what were their reasons for not taking part?
There were a lot of occasions when teams withdrew during qualification phase, but there are only 4 after qualification, and as one may guess those happened in the early years of the World Cup, when it still needed some advertisement:
As a result of Anschluss, i.e. annexation of Austria into Nazi Germany, Austria had to withdraw from the tournament despite qualification.
Scotland was granted a berth in the World Cup final phase as runners-up of British Home Championship, but their federation secretary decided that Scotland would have had to win the Home Tournament if they were to take part.
Turkey also withdrew, citing financial problems and the cost of travelling to South America.
After the draw, the Indian football association AIFF decided against going to the World Cup, citing travel costs (although FIFA had agreed to bear a major part of the travel expenses), lack of practice time, team selection issues and valuing the Olympics over the FIFA World Cup.
Although FIFA had imposed a rule banning barefoot play following the 1948 Olympics where India had played barefoot, the Indian captain at the time, Sailen Manna, claimed that this was not part of the AIFF's decision.