premise: In September that year, Callaghan had raised the possibility that Britain would face pressure to withdraw troops from West Germany in a telephone conversation with Gerald Ford, then president of the United States.
  According to a transcript, Callaghan told Ford he was "deeply concerned" about what was happening to the pound.
  "If our sterling balances go and there is no protection, I can only say to you, Gerry, that we would be faced with political decisions that would change our whole attitude as a Western partner," Callaghan said.
  He added that Britain was spending 600 million pounds a year in West Germany and that some might question why this should continue, given the domestic situation.
  "We are putting great things at risk if this pressure on sterling continues," he added.
   What led Callaghan to be deeply concerned?
hypothesis: withdraw troops from West Germany