Every historical event is caused. All historical events have consequences. From these likely premises, it is possible to argue that the most important war ever fought was the result of an obscure dynastic conflict which escalated to global dimensions and prepared the way for the world as we know it today.
In 1735, Habsburg emperor Charles VI obtained from France the recognition of his daughter Maria Theresa as Holy Roman empress. There had never been a Holy Roman empress because of an unwritten but ancient norm that restricted among the Germans the inheritance of land to males. When Charles died (1740), Frederick the Great of Prussia saw a golden opportunity to aggrandize his kingdom at the expense of Austria: he invaded Silesia and offered Maria Theresa recognition as empress in exchange for the cession of the land he had taken. Other Germans elected a male emperor and Maria Theresa, who was not easy to boss around, decided to fight. At the end of the war (1748), the Prussians held Silesia by force but not by treaty. The French, who originally had been allies of Prussia, switched sides becoming Habsburg allies, which in turn made the British side with the Prussians.
In 1756, Prussia pounced again. It was during this war that Frederick became known as the Great, not so much for what he obtained as because he survived a war against the French, the Austrians, and the Russians, a feat he accomplished by the good fortune that Russian empress Elizabeth died and was succeeded by Peter III, who admired Frederick and got Russia out of the war. In the end Prussia got to keep Silesia and the Habsburgs, male or female, were recognized as legitimate inheritors in perpetuity to the crown of the Holy Roman Empire. As this privilege lasted only until 1806 and Silesia was of minor strategic importance and eventually became part of Poland, the European war was of little consequence. The overseas war is another matter. Britain entered the war not to help Prussia but to spite France, its traditional enemy since the middle ages--at no time after the Hundred Years war (14th-15th centuries) had the French and the British ever been allies--and in the 18th century Britain and France were mortal rivals in India and in North America. Britain had the larger navy and wanted to demonstrate it. It did so splendidly. And it also conducted land operations with skill and the advantages that come with supremacy in the sea.
From the Seven Years war to the First French Republic
This war is known as the Seven Years war (1756-1763). By the time it ended, Britain had defeated the French in India twice and the young British Gen. James Wolfe broke the French ranks lined up before Quebec commanded by the veteran Joseph de Montcalm. The battle was so furious both Wolfe and Montcalm died of gunshot. The French colonial empire was in tatters; the British instead could proceed to gobble India bit by bit without European interference and in North America they joined Canada to their prosperous Atlantic seaboard colonies.
The story hardly ends there. Naval supremacy made it possible for Britain to accumulate the greatest empire in history. It made the French thirst for revenge. When the American colonies, which had not incurred expenses to get the French and their Amerindian allies off their backs, became mutinous over various levies sanctioned in the British parliament, France was eager to help. As soon as the colonists showed that they could hold their own against the British, as they did in the battle of Saratoga (1778), New York, France declared war on Britain and French aid in significant quantities began arriving. It was the French blockade of Chesapeake bay, which was possible because the British navy had too many things to do at once, that made possible the victory at Yorktown peninsula and American independence.
In the process of getting back at the British, the French got heavily in debt. The only way to balance the budget was to raise taxes, but the opposition to this in the local parlements was so stout that Louis XVI was forced to convoke the largest parlement of all, the Estates General, which in principle should have made the lesser bodies submit, although, as it hadn't met since the early 17th century, its powers were untested. Unfortunately for the king, the Estates General was dominated by the middle classes (the third estate), who were eager to rule themselves rather than to aid the amiable Louis out of the fiscal morass. After declaring its revolutionary aims, the National Assembly gave way to an elected Legislative Assembly, which was replaced by the more radical National Convention. The king, who was conspiring with French emigrés and with his father-in-law, Francis II of Austria, made a dash for Germany but was caught at Varennes, brough back to Paris, and eventually tried and beheaded (January 1793). The first French republic was born as hardly an incidental if not a direct result of the Seven Years War.
The republic ended when Napoleon was made consul for life after the dissolution of the regime known as the directorate. It was during the Napoleonic period that British-French enmity reached a climax. After variously besting German armies, Napoleon dissolved the Holy Roman Empire and did very much as he pleased in Germany. The British finally destroyed the vestiges of French naval power and made strategic alliances on the continent to contain Napoleon. Napoleon's final defeat was achieved by the Duke of Wellington with the indispensable support of Prussia, which had been reborn after Napoleon's debacle in Russia. With Napoleon's fall, Britain disengaged from the continent but remained firmly committed to the principle of the balance of power.
The end of French and Austrian hegemonies on the continent led to the rise of Prussia and the emergence of Germany as the greatest land power in Europe. The French Third Republic made an alliance with Tsarist Russia to keep Germany in check. Britain took a cold, hard look at the continent and decided that France was weaker than Germany, thus a cordial understanding between the two countries was born, their first bilateral alliance ever. Britain's concern was still basically the balance of power and it made clear that it considered Belgium's neutrality inviolable.
From World War I to the rise of Hitler
Germany had an alliance with Austria, which was Russia's rival for influence in the Balkans. Pan-Slavism was particularly strong in Serbia and when a Serbian assassinated the Habsburg heir, Austria-Hungary (as the Austrian Habsburg empire was known) declared war on Serbia. When Russia mobilized, Germany declared war on Russia and France declared war on Germany. The German strategy against France involved an invasion of Belgium, which caused Britain to declare war on Germany. World War I was on.
The western front was a war of attrition with a line of trenches extending from the English channel to the Swiss border. In the east there was more mobility, but the Russians, although they contained Austria-Hungary, were consistently being defeated by Germany. The toll on Russia was insupportable and in 1917 the Bolsheviks took power and got out of the war early in 1918. Germany could move entire armies to the western front. But in the meantime events had taken a decisive turn in the balance of forces.
Unrestricted submarine warfare was Germany's only defense against the British naval blockade. American shipping was a primary target and the USA in 1917 declared war on Germany. By 1918, German reinforcement of the western front was more than compensated by the arrival of American troops. Germany attempted one final powerful offensive against the Allies in the second battle of the Marne. It was contained and the Allies' counter-offensive broke through the Hindenburg line, Germany's last line of defense. Although Germany was not invaded, its situation was fraught and it capitulated in November.
Capitulation was not outright defeat and German nationalism soon began to gain ground until it became incarnated in Nazism, which, under Adolf Hitler, assumed total power in Germany. The British-French alliance held, but when Germany started World War II, France was defeated and Britain was left to fight alone until the USA, attacked in the Pacific Ocean by Japan (a German ally), entered the war on the side of Britain. The war ended with the unconditional surrender of Germany and the atomic bombings of Hiroshima and Nagasaki which brought Japan to its knees.
Thus, a minor dynastic war led to the Seven Years War, from which stemmed: British world naval supremacy, American independence, the French revolution, Napoleon's short-lived European empire, the rise of Germany, and two world wars. Arguably, even the Post-War was partly the result of the Seven Years war. Because of the related sequence of major events that ensued upon the direct and indirect results of that war, it should be rightfully considered the first real world war in history, and even possibly the most important.