Others have no idea when either of the World Wars took place and even struggled to name the year the most recent conflict in Iraq began.
It also emerged that almost one in five learnt all they know about Britain's war history from TV programmes or Hollywood blockbusters - and two thirds claim there are conflicts they are only aware of after seeing them unfold on screen.
The stats emerged in a study by Universal Pictures (UK) to mark the release of a special range of DVDs in support of The Royal British Legion.
The Royal British Legion's Director of Fundraising Charles Byrne, said:
"With this year's 70th commemorations of D-Day and the 100th Anniversary of the start of the First World War, we hope more young people will be engaged by key events in military history.
"Encouragingly, a lot of young people say their knowledge of military history comes from school, and we know the Legion's free school's pack on Remembrance and History reaches 2 million young people in the UK every year.
"We can also see that conflicts shown in TV and films make a big impact so would encourage everyone to pick up a DVD in this special range."
The study of 2,000 18-35 year olds found that almost half of those polled don't know that D-Day marks the Normandy Landings during World War Two, with more than one in five mistakenly believing it marks the end of the First World War.
One in four also struggled to name 1914-1918 as the dates of World War One, while one in five had no idea the Second World War took place from 1939-1945.
Even recent conflicts from the current generation leave many stumped, with just half saying they have heard of, or know about, the War on Terror.
More than a quarter of those surveyed were totally unaware of the war in Afghanistan, with even fewer - a third - saying they knew about the most recent Iraq war.
And despite it taking place in living memory for all those polled, just 40% knew that the Iraq war started in 2003.
One in two were also unaware VE Day, which took place last week, is celebrated to mark the end of World War Two in Europe.
Instead, one in ten thought it was the day the Blitz came to an end.
There was also a large number who admitted to knowing little or even to have never heard of the Cold War and Vietnam.
The study also found that while 45% say most of the knowledge they do have on the nation's military history comes from their time at school, almost one in five (18%) picked up what they do know from TV programmes or Hollywood films.
Less than one in twenty got their knowledge from talking to older grandparents or relatives.
And almost four in ten have NEVER spoken to older relatives or friends about their experiences living through military events in our past.
Universal Pictures (UK) are releasing a new range of specially packaged DVDs, about military conflicts past and present from 12 May - 50p from each DVD sold will be donated to The Royal British Legion.