Maria Theresa had never given up hope that she would recover Silesia; and as all the neighbouring sovereigns were bitterly jealous of Frederick, and somewhat afraid of him, she had no difficulty in inducing several of them to form a scheme for his ruin. Russia and Saxony entered into it heartily, and France, laying aside her ancient enmity towards Austria, joined the empress against the common object of dislike. Frederick, meanwhile, had turned towards England, which saw in him a possible ally of great importance against the French. A convention between Prussia and Great Britain was signed in January 1756, and it proved of incalculable value to both countries, leading as it did to a close alliance during the administration of Pitt. Through the treachery of a clerk in the Saxon foreign office Frederick was made aware of the future which was being prepared for him. Seeing the importance of taking the initiative, and if possible, of securing Saxony, he suddenly, on the 24th of August 1756, crossed the frontier of that country, and shut in the Saxon army between Pirna and Koenigstein, ultimately compelling it, after a victory gained over the Austrians at Lobositz, to surrender. Thus began the Seven Years' War, in which, supported by England, Brunswick and Hesse-Cassel, he had for a long time to oppose Austria, France, Russia, Saxony and Sweden. Virtually the whole Continent was in arms against a small state which, a few years before, had been regarded by most men as beneath serious notice. But it happened that this small state was led by a man of high military genius, capable of infusing into others his own undaunted spirit, while his subjects had learned both from him and his predecessors habits of patience, perseverance and discipline. In 1757, after defeating the Austrians at Prague, he was himself defeated by them at Kolin; and by the shameful convention of Closter-Seven, he was freely exposed to the attack of the French. In November 1757, however, when Europe looked upon him as ruined, he rid himself of the French by his splendid victory over them at Rossbach, and in about a month afterwards, by the still more splendid victory at Leuthen, he drove the Austrians from Silesia. From this time the French were kept well employed in the west by Prince Ferdinand of Brunswick, who defeated them at Crefeld in 1758, and at Minden in 1759. In the former year Frederick triumphed, at a heavy cost, over the Russians at Zorndorf; and although, through lack of his usual foresight, he lost the battle of Hochkirch, he prevented the Austrians from deriving any real advantage from their triumph, Silesia still remaining in his hands at the end of the year. The battle of Kunersdorf, fought on the 12th of August 1759, was the most disastrous to him in the course of the war. He had here to contend both with the Russians and the Austrians; and although at first he had some success, his army was in the end completely broken.