At length, on 11 April the British formalised their alliance with Prussia in the Anglo-Prussian Convention, in which they committed to provide Prussia with a subsidy of £670,000 annually (equivalent to £ million in ) and to make no separate peace, as well as deploying 9,000 troops to reinforce Prince Ferdinand's army in the Rhineland. Frederick decided that the time had come to invade Moravia and seize the fortified city of Olmütz, as he had planned the previous year, as soon as the last Austrians could be driven from Silesia. Schweidnitz, the last Austrian-occupied stronghold in Silesia, surrendered on 16 April, after which Frederick led a field army into Moravia, reaching Olmütz on 29 April and besieging it on 20 May.