The war started in North America. Two years later the French launched the first attack in Europe (resulting in Byng's infamous defeat at the Battle of Minorca) the British then secured an alliance with the Prussians who then launched a preemptive attack on Austria's ally Bavaria and other European powers joined in and it spread to other colonies including the third Carnatic War.
Needs context.
The situation immediately after the Peace of Aachen in 1748 was...volatile, to say the very least. Austro-Prussian hostility was virtually axiomatic; Friedrich wanted to retain his gains from the war, if not expand them with conquests in Bohemia, and Maria Theresa and her ministers (esp. Kaunitz) wanted to recover Silesia and reduce Prussia to subservience. Furthermore, unlike the previous set of wars, Russia was almost certainly going to be a cobelligerent. In the 1740s, a Russian government mostly sympathetic to Austria had been distracted by internal political intrigues and by a war with Sweden; in the 1750s, Russian policy was more coherent and focused. The preeminent Russian statesman of the moment, Bestuzhev, was intent on a preventive war against Prussia and wanted to have one at the earliest opportunity.
Enter the Anglo-French colonial war. British policy makers well knew that the French had been able to recoup their colonial losses in the 1740s by attacking British possessions and allies on the Continent. In order to keep this possibility from rearing its ugly head, the new Newcastle government began concluding a series of agreements with European powers with the aim of neutralizing Germany and protecting the British-owned Electorate of Hanover.
The first agreement, in the fall of 1755, made sense: a subsidy arrangement with Austria - which was still Britain's ally - and Russia, which was Austria's. Britain would pay for a sixty thousand man Russian army to protect Hanover from Prussia, a subvention that, the British expected, would all on its own be sufficient to keep the Prussians from making trouble in Germany. Austria had made this a precondition of any renewal of the British alliance, and the British desperately wanted to renew the alliance so that Austria could once again put pressure on France from the Low Countries. The benefits for Austria and Russia were obvious: they would effectively have their war of revenge on Prussia bankrolled by the British.
Hearing of this disaster, Friedrich soon got into contact with the British on his own. Reasoning that the British would prefer to have zero chance of having to fight in Germany instead of threatening Prussia with potential war, he got the British to agree to the Convention of Westminster in January 1756, whereby Prussia and Britain both undertook not to attack each others' territory. With this maneuver, Friedrich believed he had bought himself extra time, because surely the Austrians and Russians wouldn't attack Prussia without support - and a reconciliation with France, the only possible alternative suitor, would be practically unthinkable.
It apparently wasn't unthinkable. Kaunitz had already begun to sound Versailles out in the summer of 1755, but the negotiations hadn't really gone anywhere before the Convention of Westminster bombshell hit. Then things began to move
very quickly. France, seeing that it could not rely on Prussia to invade Hanover, decided not to renew the Prussian alliance in February 1756. (It was due for a re-up in June.) Then, in May, the French and Austrians agreed to a package of nonaggression, subsidy, and defensive agreements collectively referred to as the First Versailles Treaty.
Although some French diplomats clearly saw the First Versailles Treaty as the green light to an Austro-Russian invasion of Prussia, others probably considered it to be a bargaining chip in the British war. Technically, war had not been declared yet, and the fighting remained restricted to the North American colonies. Britain had been pursuing efforts to localize the conflict, and it's not unreasonable to see the First Versailles Treaty in the light of French attempts to do the same: by using the treaty as a bargaining chip in negotiations with Britain. If this was France's goal, though, it was very poorly executed - or perhaps nobody bothered to tell the military. After the aforementioned attack on Minorca, Parliament escalated the crisis and declared war in late May.
Meanwhile, Russia underwent the same sort of ministerial changes that had been the hallmark of the 1740s. Bestuzhev had staked a great deal on his anti-French policy, and when Britain signed the Convention of Westminster he was badly undermined. It took some time for him to be removed from his position, but in the interim, a collegiate decisionmaking system was established that gave more power to a Francophile party led by the Shuvalov brothers and including Bestuzhev's archenemy Vorontsov. Yet this ministerial shuffle meant little in terms of foreign policy because both Bestuzhev and Vorontsov firmly stood by the imperatives of the Austrian alliance and the Prussian war. So although Russia felt the effects of this Diplomatic Revolution at the top, its own diplomacy remained the same.
In fact, preparations for a Prussian war actually
accelerated with Vorontsov's rise. Even before the First Treaty of Versailles, Russia and Austria had begun to mobilize their armies preparatory to an offensive in the summer of 1756. Mobilization didn't mean the same thing as it would a hundred years later, but it was still a colossally important step toward war, and unmistakable for any other purposes. It was also difficult to organize and extremely expensive. For both of these reasons, Kaunitz tried to put the brakes on midway through and hold the offensive over until 1757, when he believed Austria would be better prepared. The Russians didn't really comply, but eventually the committee started to back off slightly, slowing down the war preparations. Bestuzhev, however, disapproved, and leaked information about the Russian mobilization in order to light a fire under the Prussians. If he could not make his own war in 1756, Bestuzhev would try to get the Prussians to start the war and have it early anyway.
Friedrich, of course, was horrified at the extent of Austro-Russian preparations, especially since he didn't know of Kaunitz's belated efforts to delay the war until 1757. Even if he had, though, that would have been no less an imperative to strike quickly, before the forces gathering against him could concentrate and overwhelm Prussia's armies. So by July 1756, he was determined to break the ring of encircling powers. Naturally, the means by which he chose to do so were embarrassingly high-handed and extremely crude to say the least; believing that the Austro-Russian preparations embraced Saxony as well, he chose to invade that country first, to prevent the Austrians from using it as a springboard to Berlin. Unfortunately, Saxony was not actually a part of the alliance and had every intention of sitting this one out. Oops. Enter seven years of Saxony as the primary battleground of Germany.
Still, it's hard to credit the accusations of black-yellow diehards who condemn Friedrich for his "rape of Silesia" in 1740 and for his preventive war against Saxony in 1756 while at the same time passing over the
Austro-Russian preparations for war without a murmur of moral outrage. These are the same sorts of people who criticize the contingents of troops supplied to Austria by the imperial fiefs for being almost all useless while conveniently passing over Austria's efforts to use the war to expand central warmaking authority in the Reich. The
character and
nature of Friedrich's diplomacy was no more or less morally repugnant than anybody else's in Europe at the time; he was simply the most successful practitioner of that diplomacy.
At any rate. The Anglo-French conflict - and the Diplomatic Revolution that it spawned - was indeed the
proximate cause of the Seven Years' War. And since it was the Ohio River crisis and the Battle of Fort Necessity that sparked that conflict, one can reasonably draw a line between George Washington and the Battle of Rossbach. But the
ultimate cause was the refusal of the Austrian, Russian, and Prussian governments to consider the Treaties of Aachen the basis for a lasting peace. Less the Anglo-French conflict, the Austrians and Russians would have still launched a war to conquer Silesia and break up the Hohenzollern monarchy, or Friedrich would still have launched a war to try to forestall that. The war would simply have happened in a different - maybe dramatically different - way.