What is a speculative attack on a country’s currency?

What is a speculative attack on a country’s currency?

What is a speculative attack on a country’s currency? Why may a central bank be unable to maintain an overvalued currency, as, for instance, the Bank of England was unable to maintain the overvalued pound in 1967? Why may a central bank be unwilling to maintain an undervalued currency, as, for instance, the Bundesbank was unwilling to maintain the undervalued deutshe mark in 1971?