Though not exactly the same, if you cut a Gordian Knot it is because you want to cross a Rubicon and when you cross a Rubicon you put behind you a dilemma that you had to solve. When Alexander the Great invaded Anatolia he reached Gordium and was shown a knot that would allow whoever untied it to conquer Asia. Alexander unsheathed his sword and cut the Gordian Knot. Julius Caesar, who envied Alexander, tried to emulate him with the conquest of Gaul. While he was away from Rome, Pompey and the senate turned against him and ordered him to stay away with his legions. When Caesar reached the Rubicon, he had to decide whether to obey or defy the order. He chose defiance and crossed the Rubicon. Incidentally, the Rubicon is a stream that flows into the Adriatic north of Rimini, where one would not normally be arriving on a return from France, which might mean that Caesar, unlike Alexander, might have hesitated.

Islamic states had a tendency to frailty because any leader could found one by occupying a political base and having his name read in the kutba or Friday prayers. Islam was initially an empire. When the Abbasids overthrew the Umayyads, the empire went to pieces. The Ottoman Turks were the only people who were able to reconstitute it. Their secret was that instead of remaining in Anatolia and fighting other Turks they crossed the Hellespont and began conquering the Balkans. In the Balkans, they formed a corps with Christian children brought up as Muslim warriors known as the janissaries. These soldiers knew only one master, the Ottoman sultan, and they lived exclusively to fight for him. As they were converted to Islam, they might have fallen into the kutba vicious circle but they didn't and they loyally and efficiently constituted the Ottoman Empire, so in a way crossing the Hellespont was like crossing the Rubicon.

The Catholic Kings, Ferdinand of Aragon and Isabella of Castile, captured Muslim Granada and brought all of Spain under Christian rule. At the time, the head of the inquisition in Spain was the relentless Tomas de Torquemada. It was Torquemada's belief that Moors and Jews weakened the kingdom. His zeal was such that he persuaded the joint monarchs to emit a decree by which Jews either converted to Catholicism or left Spain (1494). Exactly how many converted is not known but countless Jews left, depriving Spain of many of its most talented and productive subjects. Spain's might lasted for a century or more, but the expulsion of the Jews was an enervating watershed and Spain in time decayed more than it had ever flourished.  

When Hernan Cortes landed in Mexico, neither he nor his soldiers knew what perils held the interior of the country. Cortes had heard of the Aztecs and he had reasons to fear strong resistance. His men probably had them many times over. To make sure that his army did not abandon him, Cortez burned his ships in the bay of Veracruz. This was both cutting a Gordian Knot and crossing a Rubicon.

Western Christendom was irremediably divided by the Protestant reformation. There had been threats to the authority of the papacy before which had not prospered. It took an Augustinian monk named Luther not only to challenge Rome but also to obtain the political support in Germany to make his theological discrepancies with Catholicism stick. Luther had been sparring with the church authorities about the sale of indulgences--reduction of years in purgatory for the ordinary sinners in exchange of payment--but he did not cross his Rubicon until he wrote the essence of his beliefs in the "Ninety-nine Theses" which he nailed to the church door at Wittemberg (1517). After that the die was cast and, though much was done to patch up the rift, the Reformation and Protestantism were launched.

Lithuania was formed around 1250. Poland and Lithuania were a dual kingdom from 1386 until in 1569 it was fused by the Union of Lublin as the kingdom of Poland. There were many more Poles than Lithuanians, but among the latter there were many powerful land magnates for whom Lithuania was still Lithuania. The cost of their loyalty was the liberum veto, by which any member of the nobility could obstruct laws in the Sejm or parliament. Surrounded by expansive autocracies, Poland could not raise the revenues to defend itself and the country was partitioned and disappeared in 1775. The Union of Lublin was, therefore, a form of political suicide, and no commitment is as final as that.

Louis XVI was the reigning Bourbon monarch when the French Revolution began in 1789. Since the revolution got going with the king's convocation of the Estates General, it could be said that this was the crossing of a Rubicon. But there was still another one to come that spelt the end of the monarchy (for a time anyway, because there was a Bourbon restoration in 1830). Louis had to dissemble his profound distaste for the agitation of his contumacious subjects, who not only forced him to live in Paris rather than Versailles but also made him wear what to him must have been the ridiculous cockade, the tricolor rosette that was a symbol of the revolution. When Louis finally got fed up, especially because of the nagging of his Austrian wife Marie Antoinette, he cut a Gordian Knot by deciding to flee France with his family to Germany in 1791. The royal couple were recognized in the village of Varennes, near the border, and returned to Paris, where the Legislative Assembly tried and sentenced them to the guillotine. Louis was executed in January 1793 and his wife nine months later. As regicide was about as heinous a crime as could be committed then in Europe, the more extremist French revolutionaries had no compunctions about instituting later the Reign of Terror. They had crossed a Rubicon when they beheaded Louis.

Americans have crossed various Rubicons, one of which concerned Japan, a country which later did a crossing of a Rubicon which concerned America. It was in Boston that in 1775 began the American war of independence. When British troops were ordered to put down rebels in neighboring Lexington and Concord, they were met by volleys and were forced to fall back on Boston. After that the future USA were in a general state of insurrection which the British could not suppress. When in 1865 Abraham Lincoln became president the slave states started seceding from the union. Fort Sumter, facing Charleston in South Carolina, was manned by Union forces, which were given an ultimatum to surrender. Lincoln had a problem on his hands and he cut a Gordian Knot and crossed a Rubicon when he gave the order for Sumter to resist. The civil war was on.

In 1853, commodore Matthew Perry had entered Tokyo harbor with two frigates. The Japanese ordered him to go away, but as they had no naval artillery they could not prevent Perry from landing in their country. Japanese history was changed forever with Perry's crossing of a Rubicon. Japanese resistance to western influence crumbled when its antiquated forts in Kyushu were bombarded by European gunboats. Japan then crossed a Rubicon when instead of dragging its feet, the way the Chinese tried to do, it chose to modernize its economy and its armed forces.

Lenin had been preaching revolution for Russia since the late 19th century. When tsarism was abolished in 1917, Lenin returned as leader of the revolutionary Bolsheviks. There were two occasions when the communists could have lunged for power, but Lenin demurred. The provisional government headed by Alexander Kerensky proscribed the Bolsheviks and Lenin had to leave St. Petersburg to escape arrest. He returned from exile and boldly called for the overthrow of the government. In a little over a month the Bolsheviks were in power.

The Japanese entered World War I on the side of the Allies, although more than the defeat of Germany what they pursued was aggrandizing in China. For a time they temporized, but after 1931 Japanese imperialism went rampant. America was alarmed and when the Japanese occupied Vietnam, across from the American Philippines, Washington decreed an embargo on the export of fuel to Japan and froze Japanese assets. This was in November 1941. The Japanese had a dilemma on their hands, which they solved by attacking Pearl Harbor. They crossed a Rubicon, but unlike Caesar, who won his gamble and conquered Rome, the Japanese went down to defeat in a nuclear conflagration. Be it said in passing that Caesar's crossing of the Rubicon eventually resulted in his assassination.

Once America entered World War II, it shed its previous international isolationism and after the war it assumed the role of keeping an informal world empire to counter Stalinist expansionism. It did perhaps the most important crossing of a Rubicon in history when instead of backing down when the Soviets blockaded Berlin it chose to undertake the massive airlift that saved the city. By the time the Korean war started, the die had already been cast.