First World War
Until the start of the Second World War it was called the Great War, due to a lack of prescience on the part of historians of the day.
The First World War ran from 1914 through to 1918 and featured the usual players in any World War: The plucky Brits, the dastardly Jerries, the French, Italians, Russians etc. America only getting a partial attendance due to late participation)(as usual).
Started because Arnold Schwarzenegger's ancestors were pretty bolshie, the whole of Europe decided to pick sides and square up. The Allies (Brits, French, Russian and US) won WW1, a trend that was to continue in World Wars since. As noted above, the Spams started their 100% record of turning up for international dust ups.
Sadly, it was a particularly wasteful and horrible war, with huge casualties on all sides (although notable in that it was the only 20th Century war where the civilian population were getting continually beasted.
Out In the Fields
Fought in the early days using Napoleonic tactics, those who made the plans seem to have overlooked the development of the machine gun and high explosive shell, which made walking slowly towards the enemy in long lines a tactic best left alone.
Many of the tactics that evolved towards the end Of the American Civil War turn up again in WW1 just as soon as the Generals realized that none of them were Wellington. The Mine and the trench became de rigeur on the front lines.
1st July 1916 was the bloodiest day in the history of the British Army when we took nearly 60,000 casualties on the first day of the Battle of the Somme. The initial artillery barrage was so massive it was heard over much of the South East of England. The damage done to units such as the Accrington Pals left its mark for many years afterwards on the towns and cities of England.
(That is, if you accept the Blackadder Goes Forth series as a detailed and thoughtful interpretation of the historical evidence and are willing to ignore some rather damning evidence.)
There is another version, that goes a bit like this:
The tiny BEF did brilliantly in the opening months of the war, buying time to get the Territorials mobilised. They in turn held the line until 1916, while Lord Kitchener got on with the job of breaking with history, and fielding the first ever mass British Army, and the associated hardware they would need to win the war.
In the meantime, officers and NCOs who had been weaned on small-cal skirmishes against technologically inferior forces around the globe, got to grips with the complexities of industrial warfare. Ultimately, Kitchener's Armies (volunteers all, until conscription kicked in in 1916) got their first blooding in the Somme battle - in the four and a half month course of which they came to equal or outmatch the Germans in the field.)
By the time of the Passchendaele battle in late 1917, this mass army of amateur soldiers had repeatedly showed it could outclass the Boches. Its morale remained unbroken throughout the war - in marked contrast to the French, the Russians, the Italians and ultimately the Germans - despite the most appalling battlefield conditions.
In March 1918, the Hun launched a massive last-ditch offensive (Der Kaiserslacht) which in a matter of weeks, drove the British Army back over ground they had struggled for months to seize and hold. Sadly for the Germans, their advance exposed their sparsely-rationed troops to the vast cornucopia of food and drink piled up in the British supply lines. As hundreds of shagged out, footsore and war-weary German soldiers got stuck into these delights [I'm not making this up, honestly], their officers lost control and the advance petered out.
Thereafter, the Brits dusted themselves off, sorted themselves out, and with a bit of help from the French and the Septics on their right flank, began an advance in August 1918, that in the 100 days to 11 November, took them almost into Germany
The Hun finally recognised that he'd had his chips, the Kaiser went into exile in Holland, and - with latecomer Spam President Woodrow Wilson taking the lead - the Germans managed to avoid a proper defeat by negotiating an Armistice (while civil war was raging on the streets of Berlin)and submitting to punitive reparations, that would pave the way for Round 2 - or the Second World War, as it is now known.
Although much of the attention focuses on the trench warfare of the Western Front, campaigns were conducted in many other areas, such as the Eastern Front (where the Tsar's insistence upon leading his troops to slaughter lead to his ultimate fate), the infamous attack at Gallipoli and the use of guerilla tactics in Arabia by TE Lawrence.
The war ended in 1918, followed by the Treaty of Versailles which many would argue set the course for WW2
In a memorable television programme in 2007 by the BBC interviewed some of the last British survivors of the First World War, this has led to the the suggestion that a memorial service or state funeral should be arranged when the last man standing passes on.
This would constitute a fitting tribute to The Last Tommy from a grateful nation.