In the year 984 a Viking mariner bound for Greenland from Iceland, blown off course in a storm, drifted to an unknown land. There he sheltered as best he could until favorable winds rose to blow him home again. His name was Bjarni Herjolfsson — a trader and a pirate. To be one was to be the other. The moral lines were blurred.

He would have laughed to think of us 1,000 years later remembering his commonplace little sailor’s mishap, which meant nothing to him but marks for us Europe’s first known contact with the continent that 500 years later was named America.

What bearing, it may be asked, has this on Japan?

None.

Let’s proceed anyway and see what happens.

Bjarni himself seems to have been a prosaic, incurious man, but other Vikings of bolder imagination were stirred to investigate further. Sailing west, they effected a landing, founded a settlement (in modern-day Newfoundland) and traded with the natives — mostly furs for milk, the Icelandic Sagas report. The natives — centuries later to be mistakenly called Indians — knew nothing of domestic cattle. Milk to them was a delicious novelty.

The settlement foundered. Relations with the natives soured, the Vikings quarreled among themselves, one clan laid a curse on another, and it went downhill from there. By 1020 the adventure was over. America-to-be faded back into the mists. Its time was not yet.

What if Bjarni had drifted to Japan instead? He would have found a startlingly unique civilization at the height of its perfection — a hyper-refined aristocracy not of warriors but of poets, musicians, calligraphers, lovers; as devoid of the European virtues of courage and martial ardor as the Vikings were of elegant and indolent courtly ritual. This was Japan’s Heian Period (794-1185). What Bjarni made of terra incognita we know — nothing. What he would have made of Heian Japan is simply beyond imagining.

Eleventh-century Europe and 11th-century Japan were as ships in the night on a vast, vast ocean, neither aware of the other’s existence, as different from one another as the range of human possibilities allows. What if they had met? The image that immediately springs to mind — of European aggression, rapacity and absolute confidence in its God-given mission to Christianize and exploit the world — was far in the future; Japan as we know it would have survived the encounter. In fact — unlikely but not inconceivable because in history nothing is — Japan might have conquered; not by force of arms, certainly, but by force of culture, and crude and semi-barbaric Europe been as dazzled and awed by Japan as crude and semi-barbaric Japan had been 500 years earlier by China.

The tiny, cocooned Heian aristocracy should have been the happiest people in history. They had everything: wealth, ease, culture, peace and — the icing on the cake — the untroubled, unshakable, scarcely even conscious conviction that their exclusive little world was as stable and enduring a state of affairs as anything human or earthly could be.

There’s the rub. Nothing human or earthly was. Their sense of the transience of all things was acute and painful — or rather, sad. “Painful” is too strong a word. Melancholy is the dominant mood, not only felt but cultivated. Their chief happiness, it seems fair to say, and in a way their chief cultural achievement, was a deep consciousness of the melancholy lot of even the most privileged segment of humankind.

A melancholy religious doctrine underlay the sentiment. This was “mappō,” the latter days of the Buddhist Law, an inevitable fall from grace that would begin 2,000 years after the Buddha’s death in 483 B.C. The math is obscure, but rigorous study of the sutras yielded in Japan the conviction that Year 7 of the Eisho Era — 1052 by the Western calendar — would be Year 1 of decay and dissolution. Dread was in the 11th-century Japanese air.

Europe — “Christendom” — was then in ferment. Six centuries after the fall of Rome, civilization had yet to recover. War was endemic. The fighting man was supreme. Encased in steel armor, he bravely defied, or gloriously succumbed to, hail of arrows, blow of battleax and cut of spear. Higher questions, like “What am I doing this for?” were beyond him. Such civilization as there was was almost entirely in the keeping and at the mercy of the church — which was rent by schism: Latin West against Greek East, this rite against that, this conception of Christ’s relative divinity and humanity against rival ones, this date for Easter commemoration versus another.

A growing secular bureaucracy had begun to challenge the church’s monopoly of learning. Very slowly, the state was rising out of the post-Roman rubble. Here were seeds of a long, bitter and also fruitful conflict that Japan, for better and for worse, knew only fitfully — church versus state.

Rome’s heir in the east was the Greek-speaking Byzantine empire with its capital at Constantinople, modern-day Istanbul. It had survived intact the carnage of the Dark Ages but was now in trouble, beset by Turkish and Arab Muslims. In March 1095, Emperor Alexios Komnenos appealed to Pope Urban II. Would the pope not let past clashes over doctrinal fine points be bygones and send soldiers to buttress Christianity in the east?

Urban II, newly ascended to the papal throne, saw an opportunity and seized it. His actual words have not survived. This was their purport, says historian Steven Runciman in “A History of the Crusades.”

“Let western Christendom march to the rescue of the East. Rich and poor alike should go. They should leave off slaying each other and fight instead a righteous war, doing the work of God; and God would lead them. For those who died in battle there would be absolution and the remission of sins. Life was miserable and evil here... there they would be joyful and prosperous and true friends of God. There must be no delay.”

There was none. The people, it is reported, cried as one: “Deus le volt!” — “God wills it!” — and the Crusades were launched, to dominate European history for 300 years.

Back in Japan, scarcely noticed by the indolent, self-absorbed Heian aristocrats as the 11th-century wore on, a new leaven was fermenting the social dough. Warrior clans, rough and uncouth, poor hands at poetry but formidable swordsmen, laughed to scorn by their social betters who failed to see their sun setting, were slowly gaining strength. The throne’s resolve in 1050 to chastise a rebellious warrior clansman in the far northeast, defiantly levying local taxes for his own coffers, hinged on an imperial alliance with a rival clan. Even so the struggle dragged on for 12 years.

The throne won but it was a Pyrrhic victory. The clan it allied with was the Minamoto, within a century to sweep gentle Heian into oblivion and inaugurate a warrior state that plunged Japan, in effect, into a crusade without a cause — four centuries of civil war and the glorification of killing and dying as the supreme glory of the human race. This was mappō with a vengeance.

Michael Hoffman’s latest book is “Cipangu, Golden Cipangu: Essays in Japanese History.”