I'd say that the common thread that links all four of the battles that you mention, irrespective of whether they were victories or defeats, is that they involved great national sacrifices, in the sense of the blood shed by very large numbers of ordinary citizens of Britain, France, Australia and Canada. All these conflicts involved dying in an explicitly patriotic service, and can be interpreted as reflecting, in some way, national character.

In the case of the Somme, for example, the battle was fought predominantly by members of the Kitchener's "new army", recruited from volunteers after the outbreak of the war, rather than by members of the old professional army. Of course the gigantic casualties inflicted on that army also contributed hugely to the long-lasting fame of the Somme; even now the first day of the battle, with its 60,000 casualties (one-third of them fatalities) remains the bloodiest day in the history of the British army. But, going beyond that, the fact that the Somme was fought predominantly by young volunteer "amateurs" had several effects, one of which was that the recruitment system in use at the time meant that numerous units were "pals battalions" recruited from the same areas, serving together - the numerous casualties thus ripped the heart out of their communities in ways that were not easily forgotten, and the pals system was abandoned as a result.

In the long term, at least – and particularly from the 1930s onwards, as what had once been fairly widespread enthusiasm for the war, belief that the cause was just and the outcomes ultimately worth fighting for was replaced by the cynical, war-poetry fuelled "war is hell" image that predominated today – the idea that huge numbers of ordinary young men, all friends, had gone to their deaths ill-prepared for the reality of trench warfare, and been sacrificed for very little gain, was what made the Somme the quintessential World War I battle for Britain.

Much the same can be said for Gallipoli and the Australians; that campaign saw the sacrifice of large numbers of volunteer "amateurs" as well, the impact of which was magnified both by the distance that they had travelled to die, and by the fact that they sacrificed themselves in an imperial rather than a purely national cause. But, at the same time, the fact that the Australian army had fought well - and that, once again especially over the longer term, it was possible to depict their deaths as the product of bungled British leadership – helped to make it possible to portray a defeat as a sort of victory, in much the same way that the British view of the Somme is now typically shaped by the widely-discredited 1960s Alan Clarke view that the men who died were "lions led by donkeys".

Gallipoli can also be seen as a first projection of Australian power and Australian nationhood in the international sphere, something that that campaign shares with the Canadian victory at Vimy Ridge - which, in the words of Tim Cook of the Canadian War Museum, became "a defining moment for Canada, when the country emerged from under the shadow of Britain and felt capable of greatness" in their own right. Men from all four Canadian divisions fought in the battle., and hence it involved men from all parts of Canada. Canadians have also always taken pride in the fact that the battle was planned by their own officers, who introduced new and effective tactics that had not previously been seen on the Western Front.

As for Verdun, that was very much a battle of national sacrifice in which hundreds of thousands of - again, "ordinary" - Frenchman gave their lives to prevent what might easily have been a German victory. In the French case, the fact that their troops had held on and ultimately endured in the face of almost incomprehensibly appalling conditions was particularly important because it contrasted with the much poorer performance, and the quick defeats suffered by, their armies in the Franco-Prussian and Second World Wars that book-ended the Great War.

Generally speaking, and by way of contrast, the battles that are "forgotten" (always a relative term) most quickly are those fought by purely professional armies, away from home, for reasons that have little to do with national survival or the protection of families at home - especially when those battles are lost. The second Gulf campaign falls squarely into this category, since even though the specific military objectives set were met at the time, they did not result in a clear-cut victorious peace. The fact that the war was a war of aggression, rather than one undertaken because of an enemy attack (I'm setting aside 9/11 here because the Iraqis had nothing to do with that atrocity) made it even harder to view the sacrifices made by those who took part as worth it.