'Massive air battle'

According to US military archives, the US Army Air Corps began bombing Hong Kong when it was under Japanese occupation in the fall of 1942, striking the city dozens of times.

Most of those air raids were carried out by B-24 and B-25 bombers, the former of which could carry up to 8,000 pounds of bombs.

By 1945 occasional sorties by pairs of dive bombers had developed into a more concentrated campaign, David Macri, a historical researcher and academic based in Hong Kong said.

"Hong Kong was a safe harbor, protected by anti-aircraft guns, where Japanese ships could get respite but that also made it a big target," he told CNN.

"In mid-January 1945 there was a massive air battle over Hong Kong, (American aviators) were trying to sink as many ships as they could, and that's probably where this (recently discovered) ordnance would have come from."

Forgotten history

The unearthed US ordnance has put a spotlight on an often-overlooked part of the city's history.

Among the forests and walking trails that criss-cross the hills surrounding Hong Kong, lie the almost-forgotten remains of the city's wartime history. A series of pitted concrete pillboxes and bunkers, the city's wartime defenses sit neglected in amongst the trees and vines.

They are among the last vestiges of a relatively unknown chapter of World War II, a short-lived battle for a territory which, while important to British interests in Asia, ended up being quickly ceded to the Japanese by the UK military leadership.

Despite Hong Kong's perceived importance to British interests in Asia, the city fell quickly as the Allied defenders were vastly outnumbered by Japanese troops.

Britain's wartime ruler, Winston Churchill, was dismissive of Allied prospects from holding onto the vital port.

"There is not the slightest chance of holding Hong Kong or relieving it," he said in January 1941.

Early-morning attack

The Japanese invasion began in the early hours of December 8, 1941, shortly after Japanese navy launched its attack on Pearl Harbor. It was part of a multi-pronged offensive which also saw Japan, on the same day, invade Hong Kong, Guam, the Wake Islands and British Malaya, including Singapore.

Japanese troops, mainly infantrymen, marched across the Lo Wu crossing linking Hong Kong with mainland China early that morning, supported by heavy artillery barrages.

But the efforts of the around 12,000-strong Allied contingent, including relatively inexperienced troops from Canada, and more battle-hardened British and Indian soldiers, alongside local troops and volunteer units stationed in the British territory was a doomed attempt to protect the city.

Within two days, Allied troops had been pushed back from the Kowloon peninsula to Hong Kong island as the defenses strung across the border with mainland China -- known as the "Gin Drinker's Line," harkening back to happier times when families would picnic in the hills -- were overrun by vastly superior Japanese numbers and firepower.

An American officer who was caught up in Hong Kong during those fateful weeks and later repatriated as diplomatic staff reported that the defense was ineffective, but that there no shortage of bravery from the troops, says Macri.

"They really went out and did what they could. It was a smaller garrison, 12-14,000 troops, similar (in size) to a division but an ad hoc group," Macri said.

"They were faced with a hopeless task but did a good job -- as much as could be expected. They put up a good fight."

Short-lived defense

Hong Kong fell after just over two weeks of frenzied fighting between Japanese invaders and the Allied defenders, according to archives from the Commonwealth War Graves Commission.

The Royal Air Force's minimal presence -- three outdated biplanes and a couple of seaplanes -- were destroyed in a bombing raid on the Kai Tak airfield before they could even take off.

From the island, Allied troops managed to keep the Japanese at bay for a few days before they managed to cross the harbor.

When they finally crossed, in the eastern part of the island, the Allies mounted a spirited, but ultimately fruitless defense, including street fighting -- two famous lion statues outside the Hongkong and Shanghai Banking Corporations headquarters still bear the pockmarked bullet scars of the fighting -- and battles on the island's forested hillsides.

During the 18-day battle, 10% of its defenders lost their lives; a further 20% died in Japanese POW camps in the ensuing years. As many as 4,000 civilians also lost their lives.

Brutal occupation

There was an organized -- but "amateurish," according to Kwong -- resistance to the Japanese occupation, which continued until the Japanese surrender at the end of the war in August 1945.

During the three-plus years of Japanese rule, numerous atrocities were reportedly committed by Japanese troops, including civilian rapes and murders, and the execution of surrendered soldiers.

Nurses working at a field hospital were also massacred, just a day before the armistice was signed. An estimated 10,000 civilians died as a result of the occupation, mostly from malnutrition and Allied bombing, Kwong said.

It wasn't until the weeks following V-J Day, in August 1945, that the city was returned to British rule.

With Japan's surrender, Hong Kong's citizens would have breathed a sigh at relief from the US air raids -- bombs from which sometimes went astray, endangering residents -- although their legacy continues to surface to this day.