His most significant accomplishments during his reign included his military victories, his reorganization of Prussian armies, his patronage of the arts and the Enlightenment and his final success against great odds in the Seven Years’ War. Frederick was the last Hohenzollern monarch titled King in Prussia and declared himself King of Prussia after achieving sovereignty over most historically Prussian lands in 1772. Prussia had greatly increased its territories and became a leading military power in Europe under his rule. He became known as Frederick the Great and was nicknamed "The Old Fritz" by the Prussian people and eventually the rest of Germany.
Frederick was a religious skeptic, in contrast to his devoutly Calvinist father. He tolerated all faiths in his realm, but Protestantism remained the favored religion, and Catholics were not chosen for higher state positions. Frederick was known to be more tolerant of Jews and Catholics than many neighboring German states, although he considered "most Jews as less than human."
Frederick wanted development throughout the country which was adapted to the needs of each region. He was interested in attracting a diversity of skills to his country, whether from Jesuit teachers, Huguenot citizens, or Jewish merchants and bankers. As an example of Frederick’s practical-minded but not fully unprejudiced tolerance, Frederick wrote in his Testament politique:
"We have too many Jews in the towns. They are needed on the Polish border because in these areas Hebrews alone perform trade. As soon as you get away from the frontier, the Jews become a disadvantage, they form cliques, they deal in contraband and get up to all manner of rascally tricks which are detrimental to Christian burghers and merchants. I have never persecuted anyone from this or any other sect; I think, however, it would be prudent to pay attention, so that their numbers do not increase."
In one defining respect Frederick would come to the throne with an exceptional inheritance. A Prussian population estimated at 2.24 million might not be enough to confer great power status, but it turned out that an army of 80,000 men could be. The ratio of one soldier for every 28 citizens was far higher than the one-to-310 in Great Britain, another aggressively expansionist power of this period. Moreover, the Prussian infantry trained by Frederick William I were, at the time of Frederick’s accession, arguably unrivaled in discipline and firepower.
By 1770, after two decades of punishing war alternating with intervals of peace, Frederick had doubled the size of the huge army he had inherited, and which during his reign would consume 86% of the state budget. The situation is summed up in a widely translated and quoted aphorism attributed to Mirabeau, who asserted in 1786 that Prussia under Frederick was not a state in possession of an army, but an army in possession of a state.