What began with blood and pride ended in ashes and bone. The dream of a glorious new empire run from Tokyo, in which the peoples of Asia would gratefully accept the protection of Japan’s unstoppable young warriors, culminated in August 1945 amid a storm of fire and then two blinding nuclear flashes.

Nobody knows the true casualty count of the Second World War in the East. But if we include the 15-year campaign in China – as we should – it’s likely to eclipse even Hitler’s Eastern Front. An estimated 20 million Chinese died, to which we should add at least three million Japanese, hundreds of thousands of British, American and Indian troops, plus millions more who died because of famine generated by war, and the civilians of the numerous towns and cities that were fought over, from Jakarta to Hanoi.

No war is ever civilised but the campaigns in this region were distinguished by a level of irrational cruelty that almost defies description. And they started long before Pearl Harbor. The Japanese had been fighting China in Manchuria since 1931 but when the Second Sino-Japanese War erupted in 1937 it took the conflict to a horrific new level.

Caught up in a civil war between communists and nationalists, China faced a confident and technologically superior enemy. And as city after city fell, the Japanese revealed what lay behind all its propaganda about “Pan-Asian co-prosperity and brotherhood” – namely a vicious and explicitly racist bloodlust.

During a six-week rampage in 1938 that went down in infamy as the “Rape of Nanking”, soldiers of Emperor Hirohito indulged in crimes that even now are too upsetting to recount in detail. Coming from a society renowned then as now for its discipline and decorum, something truly bestial took hold as Japanese soldiers targeted young and old, especially women and children, and inflicted upon them actsof sexual violence on a scale as spectacular as it was stomach-turning.

Burying people alive, rape with sword and broken bottle, disembowelment, the torture of children, mass drownings... There is nothing vicious and vile the human mind can imagine that wasn’t done in this city and done with a celebratory relish.

Some officers even competed to get the highest kill scores. And these atrocities took place not in the initial “hot blood” of victory but slowly and systematically during weeks of intense and unrestrained sadism. But despite these well publicised horrors we would be wrong to assume everyone in the region feared Japan. In fact, during the extraordinary weeks that followed Pearl Harbor in December 1941 – with overwhelming Japanese victories occurring almost weekly – there was widespread support for what the emperor’s navy and army were doing.

It is difficult, certainly in Britain, to acknowledge this today but millions of people who had long lived under British, French, Dutch, and sometimes American, control experienced an exhilarating, thrilling pleasure at this time. We now know how cruel the Japanese were during this war, and that some of those who celebrated their freedom from European rule would experience worse from their “liberators”. Yet that doesn’t mean that their initial thrill wasn’t real or important.

In the Dutch East Indies huge numbers took to the streets to cheer the newly arrived Japanese soldiers. Armed groups of local people hunted down and killed Dutch men, women and children here too. In the notorious “pig basket” murders, dozens of captured Australian and Dutch troops were trussed up inside bamboo crates normally used for transporting animals.

Carried into public places and displayed in the hot sun for the locals to see, the men were abused, refused water and finally thrown into the sea to drown. The daughter of one eyewitness wrote that “my father told me the trucks were driven through the town as a show to the Indonesians for the utter humiliation of the white race, finally being dumped into the harbour”.

For all their military prowess, the men who ran Japan knew that they had to win quickly, or face being out-spent and out-produced by their enemies, especially America. But despite all their early successes a knockout blow eluded them and soon the Allies began battling back.

The carrier-based showdown of Midway in June 1942 is rightly seen as a turning point, but the earlier Coral Sea carrier battle and the subsequent campaigns in both Guadalcanal and New Guinea revealed a level of toughness and resilience the Japanese had not expected. And even though the British were driven out of Singapore, Malaya and Burma amid much disarray, enough of the British Indian Army survived to form a strong defence of India.

Just like the Americans, we were soon able to resupply, retrain and learn the art of jungle fighting. By late 1942, the Japanese had reached the limit of their success and – except for their great invasion of India in early 1944 – they would be fighting on the defensive for the rest of the war.

The remorseless destruction of the Japanese military began on the sea and in the air. The rapidly expanding US Navy – and especially its little celebrated “Cinderella service” of submariners – set about ridding the region of enemy military and merchant ships with astonishing efficiency.

Because the Japanese often mixed PoWs with troops on their transport ships, some US success led to the death of Allied prisoners, such as the appalling case of the Lisbon Maru in which almost 1,000 British soldiers died after becoming trapped in the holds of the sinking ship or machine-gunned by their guards as they tried to escape.

As the US “island-hopping” campaign speeded up in the Pacific, the Japanese army displayed tenacious and frequently suicidal resilience, taking many young American lives for each victory. And it was the ferocity of island battles like Saipan and Iwo Jima, culminating in the extraordinarily tough campaign on Okinawa, that led US generals and politicians to consider using a new wonder weapon on the Japanese home islands – to avoid even more terrible battles.

The flag-raising exploits of the US Marines have been much celebrated. Less so the British-led campaign in India and Burma that delivered the most significant and decisive land victory over Japan of the entire war. Bill Slim’s 14th Army (which was 80% Indian volunteers) pioneered a new style of combined arms warfare which won the turning-point battles of Imphal and Kohima in early 1944. It then drove deep and fast into Burma the following year, out fighting and out thinking the increasingly delusional Japanese generals opposing it.

The guerilla forces of the Karen people of Burma – aided and directed by agents from Britain’s SOE – played a decisive but little-known part in this historic victory. So too did a small army of military doctors and nurses. Medical support available to Allied troops improved exponentially during the war, massively reducing once sky-high rates of dysentery, malaria and “scrub typhus”.

Japanese soldiers saw no such improvement in their care. And if they were seriously injured, they would frequently be left untreated on the battlefield whereas their British, Indian or American enemies were quickly taken to clean, well run field hospitals equipped with the very latest drugs, such as the new and life-saving penicillin.

As Hirohito’s army lost more and more territory its war crimes were revealed: the grim prison camps in which so many Allied prisoners laboured, starved and died; the forced railway gangs; the death marches; and the utterly horrific germ and chemical warfare experiments carried out on Chinese and other prisoners by the sinister Unit 731.

But surprisingly Japanese commanders could be as cruel and careless toward their own men as they were to any prisoner. Military discipline was brutal and arbitrary while generals regularly issued orders that required near (or actually) suicidal tactics. Military historian Rob Lyman has described the ordinary Japanese soldier as tough and resilient throughout the war but, as it went on, increasingly exposed to fatal danger by the pride and stupidity of his own officers. A war that began with massacres of Chinese civilians finally ended with the mass slaughter of Japanese ones in the unrelenting US firebomb raids of 1945 and then, finally, at Hiroshima and Nagasaki.

Some today call these war crimes, and there is no doubt that civilians were intentionally targeted. But in all my years writing about the Second World War, and that includes reading numerous diaries and letters and having some intense conversations with veterans, I have never once found anyone who disagreed with what the Americans did.

It seems clear the appalling deaths at Hiroshima and Nagasaki prevented many more civilian and military casualties. And as this Daily Express series will make clear, ending a war of such unparalleled brutality likely did require a shocking moment of ash and bone.

• Phil Craig is the author of 1945: The Reckoning – War, Empire and the Struggle for a New World (Hodder, £25)