Although Austria had actively prepared for a renewed war with Prussia ever since 1748, it was still taken by surprise when Prussian forces invaded Saxony on 29 August 1756.
Her alliance with France, including French subsidies had been agreed in January 1756 and the French had also entered into an agreement to fund Bavaria in March. Bavaria had suffered badly in the War of Austrian Succession and had reduced its army to 5100 Infantry and 1190 Cavalry (who only had 270 horses between them). The French subsidy allowed the recruitment of the 10,000 strong Auxillary Corps.
Equally France and Britain had been in an undeclared war since 1755 in North America. This was steadily escalating and both sides had sent fresh regular battalions to reinforce their armies on the continent. In Europe, itself, the two clashed at Minorca where a French force under the Duc de Richelieu, with 20,000 men, seized Fort San Filipe that guarded Port Mahon.
(Fort San Filipe, a classic example of the star shaped fortresses built in the early years of the Eighteenth Century)
To celebrate, de Richelieu ordered his personal chef to prepare a new dish, Sauce Mayonnaise, and the British responded by shooting Admiral Byng. Byng was blamed for being defeated despite having superior numbers (13 ships to the 12 French) when his formation was disrupted at the start of the battle.
However, the Prussian invasion of Saxony caught Austria ill-prepared (and also surprised Prussia's British ally) but not completely unprepared. Browne, at Prag, had 2 corps to hand and a third under Piccolomini nearby. Equally substantial forces were marching towards Wien from Hungary, Italy and Croatia. The hope was that the large, if ill-trained and poorly led, Saxon army would delay Frederick long enough for Austrian reinforcements to reach the front and France to enter the war In Europe.
(the forces available to Browne at, and around Prag, when the Prussians entered Saxony)