So quiet had been the preparations, that none of their enemies dreamt that the Prussians would assume the offensive, but considered that they would confine their efforts to defending the defiles into Saxony and Silesia. But this was not Frederick's idea. As spring approached, he had been busy redistributing his troops from their winter cantonment, and preparing three armies for the invasion of Bohemia. April had been a busy month for the staff, and the aides-de-camp had passed their days, and even their nights, on horseback.

At last all was in readiness for the delivery of the stroke, and on the 20th the king started from Lockwitch, facing the old Saxon camp at Pirna; the Duke of Bevern from Lousitz; and Marshal Schwerin from Schlesien; and without the slightest warning, the three great columns poured down into Bohemia.

The movement took the Austrians absolutely by surprise. Not dreaming of such a step on Frederick's part, they had prepared, near the frontier, vast magazines for the supply of their advancing army. These had to be abandoned in the greatest haste, and a sufficient amount of food to supply the entire army, for three months, fell into the hands of the Prussians. Marshal Browne and General Konigseck, who commanded the Austrian armies in Bohemia, fell back to Prague with the greatest speed that they could make.

The light irregular corps, that Frederick had raised during the winter and placed under experienced and energetic officers, pervaded the whole country, capturing magazines and towns, putting some to ransom, dispersing small bodies of the enemy, and spreading terror far and wide. Browne succeeded in reaching Prague before the king could come up to him. Bevern, however, overtook Konigseck, and greatly hastened his retreat; killing a thousand men and taking five hundred prisoners, after which Konigseck reached Prague without further molestation, the Duke of Bevern joining Schwerin's column.

The Austrians retired through Prague and encamped on high ground on the south side of the city, Prince Karl being now in command of the whole. Had this prince been possessed of military talents, or listened to Marshal Browne's advice, instead of taking up a defensive position he would have marched with his whole army against the king, whose force he would very greatly have outnumbered; but instead of doing so, he remained inactive.

On the 2nd of May, twelve days after moving from Saxony, Frederick arrived within sight of Prague. So closely had he followed the retreating Austrians that he occupied, that evening, a monastery at which Prince Karl and Marshal Browne had slept the night before. Thirty thousand men, who were under the command of Marshal Keith, were left to watch Prague and its garrison; while Frederick, on Tuesday, searched for a spot where he could cross the river and effect a junction with Schwerin. He knew his position, and had arranged that three cannon shots were to be the signal that the river had been crossed.

A pontoon bridge was rapidly thrown over, the signal was given, and the Prussians poured across it; and before the whole were over Schwerin's light cavalry came up, and an arrangement was made that the two forces should meet, at six o'clock next morning, at a spot within two miles of the Austrian camp on the Lisca hills.

All this time the Austrians stood inactive, and permitted the Prussian columns to join hands without the slightest attempt to interfere with them. Had Browne been in command, very different steps would have been taken; but Prince Karl was indolent, self confident, and opinionated, and had set his army to work to strengthen its position in every possible manner. This was naturally extremely strong, its right flank being covered by swampy ground formed by a chain of ponds; from which the water was let off in the winter, and the ground sown with oats. These were now a brilliant green, and to the eyes of Frederick and his generals, surveying them from the distance, had the aspect of ordinary meadows. The whole ground was commanded by redoubts and batteries on the hill, which rose precipitately seven or eight hundred feet behind the position. In the batteries were sixty heavy cannon; while there were, in addition, one hundred and fifty field guns.