In early 1972 the war in Vietnam was entering its final phase. The Americans were withdrawing, leaving South Vietnam on its own to face an aggressive communist North Vietnam. The communist were amassing a huge well-equipped army to overrun the South the moment the Americans left.

South Vietnamese commanders decided to block the coming invasion at the Cua Viet River. Bizarrely, they also decided not to use their own air force lest they infuriate the attacking communists and derail proposed peace talks!

As a result, a small Vietnamese force found itself alone at the strategic Dong Ha bridge trying to hold off an invasion without air support. This became the setting for one of the greatest feats of heroism in modern warfare.

Captain John Ripley had been posted by as an adviser to the Vietnamese unit holding the Cua Viet line and he was dug in with them at Dong Ha Bridge. Ripley was a top solider, the best in his profession. He had graduated from the US Naval Academy, had Marine training, served with the British Royal Marines, been in the Navy SEALS and the Army Rangers. His skills and professionalism made him invaluable and his cheerfulness and bravery lifted the spirits of the fearful Vietnamese.

Bad news came after several sleepless nights being pounded by communist artillery: they would get no air or artillery support - they were on their own. Across the river the communists were sending hundreds of Soviet-supplied tanks straight towards them. Once over the river, the attackers would slice through them and into the country below.

Their only hope was to destroy Dong Ha Bridge. And John Ripley was the only person able to do it.

Ripley eyed the massive structure, built some time before by US army engineers. It was designed to take heavy traffic and withstand artillery hits. So it would be quite a job to bring it down. But either he blew this bridge or South Vietnam was lost!

By knocking one span of the bridge off its supports he would open a forty-meter gap and render the whole thing impassable. A number of well-placed explosive charges would do it. He radioed for supplies to be dropped by helicopter and set out with two trusted Vietnamese colleagues.

A sprint across open ground under heavy enemy fire brought them to the foot of the bridge. The approach to the bridge had been closed with a tangle of razor wire to keep out saboteurs. There was no time for wire cutting so Ripley shouldered the explosives and picked his way through the vicious barrier. Soon sweat and blood poured from his body but Ripley cleared his mind of any thought except one: blow this bridge.

After a tortuous struggle he was under the bridge and able to grab a big steel girder with both hands. By swinging his legs up and keeping himself spread eagle between two girders he was able to inch forward hauling the explosives to their destination. Finally a box was in place.

But the task was only begun. Ripley calculated he would need to repeat the trip back and forth between the detonation point and his colleagues on the other side of the razor wire five or six times to place enough explosives. And do it under intense enemy fire. If he allowed his spread-eagled body to sag just a little as he inched forward he would take a bullet.

The whole prospect was enough to discourage any brave man. But not Captain John Ripley. He spent the next exhausting two hours at this nightmarish task. The physical and mental strain sapped his strength. Tank rounds slammed into the bridge around him with fierce vibrations that almost caused him to lose his grip.

His response was to focus all the more on his job. The fate of a whole nation was in his hands. He found himself whispering over and over: "Jesus and Mary, get me there! Jesus and Mary, get me there!"

Finally all the charges were in place but now a terrible realisation dawned on him. He had no time fuses!

Procedure was always to set a second back-up detonation by running electrical cable and this cable was in one of the bags. He prayed it would work as he inched along uncoiling the wire. Two hours later, with hardly strength left to stand, Ripley rejoined his two friends for the dash back across open ground. He would unwind the rest of the wire on the run. The sprint brought them back to starting point but now there was another setback: there were no crimping pliers to fix the wires to the detonators! It would have to be the old trick of doing it with one's teeth. One slight error and he would get his mouth blown off.

And then they found the bag of time-delay fuses nearby! Not one for taking chances, Ripley decided to go back through the razor wire and place the time fuses.

Soon he was once again under the bridge. He stared down at the swirling waters of the Cua Viet below him and wondered if his strength would last and if his friends could finish the job if he did not make it. And he prayed. From his parched mouth came the hoarse words, over and over again, "Jesus and Mary, get me there!"

That last superhuman effort paid off. The wire system failed but the time fuses worked and with a deafening bang a span of the bridge plunged into the river.

Captain Ripley's heroic self-sacrifice halted the invasion and gained valuable time for South Vietnam. Unfortunately the political leaders were not men of his calibre and what was won so hard on the battlefield was squandered on negotiation table. Three years later the South collapsed, thousands were executed, tens of thousands were imprisoned, hundreds of thousands of "boat people" fled the country in rickety boats and today Vietnam still languishes under a communist tyranny.