Peter III dies at Ropsha, at the hands of Alexei Orlov (younger brother to Grigory Orlov, a court favourite and a participant in the coup) on July 17, 1762—eight days after the coup and just six months after his accession to the throne.

Historians find no evidence for Catherine's complicity in the supposed assassination.

At the time of Peter III's overthrow, other potential rival claimants to the throne exist: Ivan VI (1740–1764), in close confinement at Schlüsselburg, in Lake Ladoga, from the age of six months; and Yelizaveta Alekseyevna Tarakanova (1753–1775).

Ivan VI will be assassinated during an attempt to free him as part of a failed coup against Catherine: Catherine, like Empress Elizabeth before her, gives strict instructions that he is to be killed in the event of any such attempt.

Ivan will be thought to be insane because of his years of solitary confinement, so might have made a poor emperor, even as a figurehead.

Catherine, though not descended from any previous Russian emperor of the Romanov Dynasty (she descends from the Rurik Dynasty, which preceded the Romanovs), succeeds her husband as empress regnant.

She follows the precedent established when Catherine I (born in the lower classes in the Swedish East Baltic territories) succeeded her husband Peter the Great in 1725.

Historians debate Catherine's technical status, some seeing her as a regent or as a usurper, tolerable only during the minority of her son, Grand Duke Paul.

In the 1770s, a group of nobles connected with Paul (Nikita Panin and others) will consider a new coup to depose Catherine and transfer the crown to Paul, whose power they envisage restricting in a kind of constitutional monarchy.

However, nothing will come of this, and Catherine will reign until her death.

Catherine will extend the borders of the Russian Empire southward and westward during her reign to absorb New Russia, Crimea, Northern Caucasus, Right-bank Ukraine, Belarus, Lithuania, and Courland at the expense, mainly, of two powers—the Ottoman Empire and the Polish–Lithuanian Commonwealth.

All told, she will add some two hundred thousand square miles (five hundred and twenty thousand square kilometers) to Russian territory.

Catherine's foreign minister, Nikita Panin (in office 1763–81), exercises considerable influence from the beginning of her reign.

A shrewd statesman, Panin dedicates much effort and millions of rubles to setting up a "Northern Accord" between Russia, Prussia, Poland, and Sweden, to counter the power of the Bourbon–Habsburg League.

When it becomes apparent that his plan cannot succeed, Panin will fall out of favor and Catherine will have him replaced with Ivan Osterman (in office 1781–97).

Catherine agrees to a commercial treaty with Great Britain in 1766, but stops short of a full military alliance.

Although she can see the benefits of Britain's friendship, she is wary of Britain's increased power following its victory in the Seven Years' War, which threatens the European balance of power.

Sweden, under the sway of the Cap faction, seems in danger of falling prey to the political ambitions of Russia.

It appears on the point of being absorbed into the Northern Accord sought by the Russian vice-chancellor, Count Nikita Panin.

It seems to many that only a swift and sudden coup d'état can preserve Sweden's independence.

At this juncture Gustav III is approached by Jacob Magnus Sprengtporten, a Finnish nobleman, who has incurred the enmity of the Caps, with the prospect of a revolution.

He undertakes to seize the fortress of Sveaborg in Finland by a coup de main.

Once Finland is secured, he intends to embark for Sweden, join up with the king and his friends near Stockholm, and force the estates to accept a new constitution dictated by the king.

The plotters are reinforced by Johan Christopher Toll, another victim of Cap oppression.

Toll proposes to raise a second revolt in the province of Scania, and to secure the southern fortress of Kristianstad.

After some debate, it is agreed that Kristianstad should openly declare against the government a few days after the Finnish revolt had begun.

Duke Charles (Karl), the eldest of the king's brothers, will thereupon be forced to mobilize the garrisons of all the southern fortresses hastily, ostensibly to crush the revolt at Kristianstad, but on arriving in front of the fortress, he is to make common cause with the rebels and march upon the capital from the south while Sprengtporten attacks it simultaneously from the east.

The entire revolutionary enterprise is underwritten with loans procured from the French financier Nicolas Beaujon, arranged by the Swedish ambassador to France, Count Creutz.

Toll succeeds in winning the fortress of Kristianstad on August 6, 1772, by sheer bluff, and ...

Sprengtporten succeeds in surprising Sveaborg on August 16, but contrary winds prevents him from crossing to Stockholm.

Events will soon occur there that make his presence unnecessary in any case.

The Cap leader, Ture Rudbeck, arrives at Stockholm on August 16 with news of the insurrection in the south, and Gustav finds himself isolated in the midst of enemies.

Sprengtporten lies weather-bound in Finland, Toll is five hundred miles away, and the Hat leaders are in hiding.

Gustav hereupon resolves to strike the decisive blow without waiting for Sprengtporten's arrival.
He acts promptly.

On the evening of  August 18, all the officers whom he thinks he can trust receive secret instructions to assemble in the great square facing the arsenal on the following morning.

At ten o'clock on August 19, Gustav mounts his horse and rides to the arsenal.

On the way, his adherents join him in little groups, as if by accident, so that by the time he reaches his destination he had about two hundred officers in his suite.

After parade he reconducts them to the guard-room in the north western wing of the palace, where the Guard of Honor has its headquarters, and unfolds his plans to them.

Gustav now dictates a new oath of allegiance, and everyone signs it without hesitation.

It absolves them from their allegiance to the estates, and binds them solely to obey "their lawful king, Gustav III".

Meanwhile, the Privy Council and its president, Rudbeck, have been arrested and the fleet secured.

Gustav makes a tour of the city and is everywhere received by enthusiastic crowds, who hail him as a deliverer.

On the evening of August 20, heralds roam the streets proclaiming that the estates are to meet at the palace on the following day; every deputy absenting himself will be regarded as the enemy of his country and his king.

On  August 21, the king appears in full regalia.

Taking his seat on the throne, he delivers his famous philippic, viewed as one of the masterpieces of Swedish oratory, in which he reproaches the estates for their unpatriotic venality and license in the past.

A new constitution, the Instrument of Government, is read to the estates and unanimously accepted by them.

The diet is now dissolved.