I'm going to warn you beforehand; many answers to this question will be even more subjective than a normal history question. Not only is this highly controversial in debate, and relevant to our world today - but I truly believe there are so many very applicable, good arguments to be made for a variety of 'solutions' to how the war was avoided. Add an 'I believe...' to all of the following. :)

First of all; the war could not be avoided by at least, say, 1909 at the latest. The direct collision course that Tirpitz's naval policy had set out meant that the UK was inevitably set to join with the Entente - the accords signed in 1904 were hardly binding, mind you, but the threat to the magnificent, beloved Empire was as if written in blood to the Briton people. So, Germany would had to have abandoned this undertaking, which is a issue within itself. For, while this dedication did happen in the 1900 - 1910 region, the decision for this collision of Germany and Britain was ultimately dedicated in the 1880s/1890s. Two things which gave birth to this naval arms race: Bismarck's reversal of policy in regards to a German colonial empire (and as such taking what was not taken, like South West Africa, or taking what has been taken, like British South Africa), and then the subsequent sacking of Bismarck by a naive, war-wanting Kaiser. So, here, in the 1890s, we have the base of this 1900s naval decision which was backed up by a power-monopolistic Kaiser who made rash, politically incorrect statements of a jingoistic nature - be though his naive, short-sighted attitude in reality, it certainly did not help.

So that accounts for the earliest we could have avoided the British intervention, in my opinion. If we had Bethmann-Hollwegg held power earlier instead, well maybe we could have had this more friendly reversal of foreign policy - and avoided the British - German rivalry. Do remember, Britain did not commit itself up until the very moment Belgian soil was crossed. That was what united the divided opinion of the British people, an opinion held by many of its descendants today, who are swaying away from continental commitments.

Then you come up against even more intricate arguments about British policy. Yes, they were anti-continent (Sir Henry Wilson, a junior officer, was forced to operate beyond formal commitment from Parliament while planning with the French General Staff in 1913!) but they did like a friendly continent. Surely, you could ask, wouldn't the danger to the famed 'balance of power' draw the British Empire into inevitable conflict with a paranoid, lashing Germany? It had forced the British to do so nearly a century earlier, indeed.

Two things answer to this, and lead to the next argument. For that explains the limit of British friendliness and possible avoidance of the Great War. But would there have been a war nonetheless even without the UK?

For there to be a balance of powers on the continent, they had to be of even nature and power. And, as we all know, the balance here was the Tripartite Pact and the Entente. A tangled web of alliances secured the nations of 1914 Europe like concrete in mud. So, assuming that Germany somehow took not a priority on naval construction, in instead devoted its efforts entirely into army advancement, could they had sidestepped the tricky issue of British protectionism of the Empire?

For this addresses on whether Germany could have avoided fighting Russia and France.

Now, 19th Century politics is not my speciality like the First World War. But it is my belief that the seeds of this war were planted back in 1848. For the clash of the giants of France and Germany was always going to be inevitable. France had been the superpower in Europe for centuries. It had generally faced (at a simplified, overall view of past politics) two massive coalitions in 1630 and 1806. Both times it descended into a war which ravaged Europe, forever changing the physical and social landscape of Europe. 1648 had given birth to the nation-state, and 1815 had - in the end - given inspiration to the common people. This swirled up into a more violent existence in 1648 across Europe, but particularly in Germany, the cracked, torn Germany of many centuries.

But France was still the power of the continent. The Russian bear was rising, it was industrializing and consuming the potential of its people; as was checked in the Crimea in 1854 by France, Britain and the Turks. France, meanwhile, duelled with the arch-rivals of the Austrians over the political freedom of a hopeful Italy in Lombardia.

What does this have to do with 1914? Well the chaos of 1866 to 1871 struck down the arch-rival of Austria and from the ashes of two continental wars, emerged a unified, hungering, powerful Germany. For Bismarck had not just replaced Austria as the head of the German people, but also as the sworn rival of the over-protective France. The subsequent annexation of Alsace - Lorraine, the heaping of humiliations upon the French people in 1871 and, possibly most importantly, the final death of the legend of the French army, so embellished in the flames of Napoleon's armees. Well, this bred resentment like bacteria.

It is generally agreed that the 'lost provinces' of 1871 were not a burning ember in the French national conscience. The youth had, as Max Hastings claims, essentially forgotten the past defeats. But 1871 still represented the start of this railroad in history. Or, at a pinch, 1866.

For the 'balance of power' was not a concept which existed solely in the British mind and fears. It existed very much so in European politics. For one nation to have overarching power over the rest was unacceptable, be it by design or coincidence. One nation has never gained total dominance over Europe in modern history. France definitely held the gauntlet for much of the early modern period, but it was always being challenged by the Hapsburg dynasty - both Spanish and Austrian - or the British, or even the Russians eventually.

So, that explains why the French and Germans were always set on a course of conflict, as everyone was well aware. This also explains why the UK would most likely be drawn into a war of continental protection - though their exclusion from the war was always a possibility, as I said, until the Belgian invasion. But it would be war nonetheless.

And, of course, if Germany looked the other way, it had a burgeoning Russia at its eastern, Prussian homeland gates bursting with energy. The 1905 war and surrounding revolutions had not halted the Russian bear. Its continual rise to the forefront of Europe was an accepted fact in European courts, and Germany saw itself as facing the bear head-on. As every year passed, it just added more urgency to the German politicians who pointed anxiously to the bear at the gates. This urgency, of course, translated to rising tension with France, who was at the other side of Germany trembling with suspicion and resentment of their arch-rivals.

So, no, by the general course of history, the war could not be miraculously avoided in the lead-up. A more limited war, perhaps. A later war, also possible. But war was on the tongue of every European politician as a solution to all of their problems. It would have taken a remarkable instance of the nations coming to a sensible debate and agreement to overcome this instinctive distrust. But fear moulded the course of history. The British would not commit to the continent. The Germans were ruled by a politically foolish Kaiser, and were peopled by a spirit of perceived inferiority. The French were seized by a feeling of vulnerability as well as that of the inevitable. The Russians were swirling in chaotic expansion while Austria battled the will of its own population.

There is so much more to this complex relationship of nations. But you have my rather simplified, to the point answer as to the latest the war could be 'avoided', per say. I have not delved into the tense string of ethnic relations in the Balkans - be they that of the Austrian-Hungarian empire or the breakup of the Ottomans - or the tender balance of British/Entente relations all the way up until 1914.

[Three books which answer this more clearly; Barbara Tuchman's 'Guns of August' (of course!), Max Hasting's 'Catastrophe' and Paul Ham's '1914: The Year the World Ended'.]