29 May 2016 Last updated at 00:05 BST
It lasted 300 days and as many as 800,000 troops were killed, wounded or went missing.

Wales are in Group D alongside Austria, Serbia, Republic of Ireland, Moldova and Georgia.
"It's a good group and you look at other groups and think it maybe could have been easier, maybe it could have been tougher," Coleman said.
"It's a positive group, there'll be some great games and I think the whole nation will be looking forward to it."
Wales were one of the top seeds for Saturday's 2018 World Cup draw in St Petersburg after a remarkable rise under Coleman to be ranked 10th in the world.
Their reward was being one of the top nine seeds which ensured Wales would avoid the likes of world champions Germany and European champions Spain, arch-rivals England and hosts Russia.
Wales avoided former World Cup winners France and Italy, who were in pot two, and instead will face Austria, who they beat 2-1 in a friendly when they last faced them in February 2013.
However, Coleman admits that the teams they will face are no pushovers.
"We're looking forward to it. There's some good teams in there, strong teams," Coleman said.
"It's never easy going to places like Georgia and Moldova. It's always very difficult, and, of course, we know the quality of Republic of Ireland and Austria.
"In the last campaign in the World Cup [2014 qualifying] we faced Serbia and were beaten heavily twice, so we know there's good teams in there.
"But what we know is in the last two or three years we've really improved as a nation and at the moment, at our strongest, we fancy ourselves against anyone.
"I don't say that flippantly because we're in amongst some good teams."
The top team from each group will qualify for the finals in Russia with the eight best second-placed teams going into the play-offs.
Wales currently top their Euro 2016 qualifying group after beating Belgium 1-0 in Cardiff last June.
They hold a three-point lead with four games remaining, needing to finish in the top two to secure their place at a major finals for the first time since 1958.
"We've got a lot of football to play in this European qualifying campaign before we get to the next one," Coleman added.
France is commemorating the 100th anniversary of The Battle of Verdun, World War One's longest battle.
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Manager Chris Coleman was content with Wales' 2018 World Cup qualifying group after Saturday's draw in Russia.