In the same year the crown of Austria fell to a woman, Maria Theresa, daughter of the late Emperor Charles VI. Charles had persuaded the chief men of Europe to agree that his daughter should follow him as he had no son, and this agreement was known as the Pragmatic Sanction. Frederick now demanded that the Empress should give up to him the province of Silesia. Silesia had once belonged to Brandenburg, but had been lost in the Thirty Years' War. Maria Theresa refused, and Frederick marched into Silesia at the head of the army which had been so well drilled by his father, and seized the province. In 1741 he beat the troops of the Empress, and in 1742 she made peace with him and gave up Silesia.

Maria Theresa had many troubles to distress her, but she was a stout-hearted woman and strove with all her might against her enemies. The Elector of Bavaria said that he ought to have received the crown of Austria, and he attacked Maria Theresa, and won such success that he was crowned Emperor in 1742 as Charles VII (1742-1745). For the next three years there was a great deal of fighting between Bavaria and Austria, each side being helped by a number of allies. Frederick, too, joined in the warfare, for he feared lest the Austrians should take Silesia again, and in 1745 he fought Maria's troops and beat them. Charles VII died, and a general peace was patched up on these terms: the new Elector of Bavaria gave up all claims to Austria; Maria Theresa gave up all claims to Bavaria; Maria's husband, Francis, was chosen as the new Emperor Francis I (1745–1765); Frederick was left in possession of Silesia.

For some years there was peace in Germany, and Frederick led a very busy life improving his Kingdom of Prussia. He looked very carefully to the finances of the state, travelled about the country to find out how his subjects lived and what he could do for them, built splendid buildings in Berlin, and kept a watchful eye on his army, for he never knew when he might need to make a stand against his enemies. Of these the most bitter was Maria Theresa. The proud and brave Empress had never forgotten the loss of Silesia, and Frederick knew that she would do her utmost to regain the province if she saw an opportunity.

In the year 1756 Frederick heard bad news. His spies reported to him that Maria Theresa had made a secret agreement with France and Russia to assail Prussia and overthrow his throne. Frederick, full of boldness, daring, and energy, resolved to strike the first blow. He at once crossed his frontiers with a powerful army and defeated an Austrian force in a battle fought in Saxony. This was the first encounter of the famous Seven Years' War (1756-1763), in which Frederick fought single-handed against the three greatest Powers of Europe, and won his title of Frederick the Great. He did not win this name because he was always the victor; far from it—he suffered many and many a bitter defeat. But he never gave in. At the moment when his foes thought they had crushed him for ever, he was busily gathering together once more the remnants of his shattered forces, weaving anew the web of his policy, marching again to offer battle to those who had considered him to be beaten once and for all.

In 1757 the war began in earnest. The ban of the Empire had been issued against Frederick, and the Emperor had declared him to be a rebel and a traitor. Frederick took very little notice of this, but the news that four powerful armies were advancing upon Prussia from four different points was very serious. Frederick struck first at the Austrians and beat them near Prague. Another army came up and Frederick in turn suffered a heavy defeat, losing many thousands of his best troops, his artillery, and his baggage. He retired into Silesia to meet further bad news: the Russians and the French had won victories over his friends. In November of the same year Frederick met the French at Rosbach, near Leipzig, and triumphed over foes three times as numerous as his own troops, and a month later he won a wonderful victory at Leuthen, in Silesia. Thus 1757 ended brightly for the Prussian cause.

In the next year Frederick was not so fortunate. It is true that he drove the French over the Rhine in the west, and beat back the Russians in the east, of his state, but he lost a great battle with the Austrians, and many of his best troops were slain. His funds, too, were running very low, though England helped him with a large sum every year, and his enemies felt certain that another campaign would crush him and end the war.

It is true enough that the campaign of the next year, 1759, was as bad for Frederick as his worst enemy could have wished. A great Russian army advanced to the Oder, and waited for the Austrians to come up, when both armies meant to march into the heart of Brandenburg. Frederick attacked the Russians and drove them back. Next he turned on the Austrians, but these were fresh and full of fight, while the Prussian troops were weary, and Frederick suffered a terrible defeat. He would now have been utterly crushed, but his enemies started to quarrel among themselves, and this gave him a chance to draw his shattered forces together. He sent out a fresh army under one of his best generals, but it was forced to surrender in a body, and the fortunes of Prussia sank to a very low ebb. Still Frederick maintained the struggle.

In the next year, towards the end of the campaign, Frederick gained a victory which made up for many of his defeats. In November he overthrew the Austrians at Torgau, in Saxony, the last decisive battle of the Seven Years' War. Still Frederick's difficulties were not at an end, for the English Government withdrew the payments which had helped him so much, and he knew not where to turn for money to pay the great expenses of the war. England, too, advised him to give up Silesia, and it seemed that Frederick would have to own himself beaten, so much was he at a loss for men and money. Then a strange turn of affairs was seen, a change which saved him at the last moment. His bitter enemy, the Empress Elizabeth of Russia, died, and her nephew, Peter, came to the throne. Peter was a great friend and admirer of Frederick, and he not only ordered the Russians to make peace with the King of Prussia, but sent an army to his aid. Thus, at a stroke, a powerful enemy became a powerful friend.

There was some more fighting, but by this time both sides were weary of the struggle, and early in 1763 a peace was made by which Frederick retained Silesia, nor was any further attempt made to wrest it from his grasp.

This remarkable war in which Prussia made so wonderful a stand against such strong enemies raised the kingdom to a high position in Europe. From that time Austria and Prussia became the two chief German states, the Protestant states looking more and more upon Prussia as their leader.

A long period of peace followed the Seven Years' War, and Frederick laboured with all his might to make good the damage which the fierce struggle had inflicted on his country. He gave the people grain to sow in their fields, he freed them from taxes where they were too poor to pay, he encouraged them to settle in districts which had been laid waste, and founded hundreds of new villages. In religious matters he held the scales equally between his subjects, treating both Protestants and Catholics alike. He was a great lover of justice, and saw to it that every man had his rights in the courts of law. In this connection there is a famous story of Frederick and a miller whose mill stood beside the grounds of the king's palace at Sans Souci. Frederick wanted to buy the mill and pull it down, and make his gardens larger. The miller refused to sell. The king offered a great sum of money, and said that he would build a new mill for him. Still the miller refused. Frederick lost his temper and declared that he would pull the mill down about its owner's ears. "No, you won't," said the miller, "if you lay a finger on my mill I'll summon you before the law courts in Berlin."

Frederick took this bold answer in good part, and was greatly delighted at the man's confidence in the reign of law in his kingdom. The mill was left untouched, and became a national monument of the respect which Frederick the Great paid to the law.

Another famous story about Frederick shows his kindness of heart. One morning he found the page who waited upon him asleep in the ante-chamber. Frederick saw a paper sticking out of the boy's pocket, and he drew the paper out and read it. It was a letter from the boy's mother, who was very poor, thanking him for his goodness in sending her money from his small wages. The king was pleased with this kind act of a good son, and slipped a handful of gold coins into the letter and put it back in the boy's pocket.

Frederick now went back to his own room and rang the bell so loudly that the page was awakened at once. The boy was so astonished to find his pocket full of gold that he went all colours. "What is the matter?" asked Frederick; and the page at once told him of the strange thing that had happened.

"Ah," said the king, smiling, "it must have been Dame Fortune who paid you a visit while you were asleep. Send the money to your mother, and tell her that from this time she will find a friend in Frederick."

Until his death in 1786 he strove to raise Prussia to a high position in Europe, and he won great success. He left his country strong and prosperous, and when he died there was great sorrow felt for the loss of "Old Fritz," as his people loved to call him.