When rumours of an Ottoman army gathering strength began to spread in the spring of 1532, the Habsburgs began to mobilise their forces against this threat. In negotiations with the estates of their own territories, with the rulers of the Holy Roman Empire and with other European potentates, they sought to win as many allies as possible. This work is devoted to the diplomatic events that ultimately led to the formation of the largest military force ever fielded against the Ottomans.