Hi Francois, just my two-penn'orth. In my view the reasons for the outbreak of WW1 were more subtle than mere sabre-rattling & "imperialism".
The early years of the 20th Century mark the end or the beginning of the end of the global influence of the old empires, Russia, Britain, Austro-Hungary & France, & the rise of the newer indusrialised nation states, Germany & USA. It was not realised at the time, but with hindsight it was obvious that this would happen. It might be easier to examine each country's view at a time.
The continued cost of the British Empire was becoming a millstone around Britians neck, & it is often forgotten that there was a recession in the early years of the new century. Britain had just fought the Boer War, the cost of which in monetary terms or in casualties had far exceeded expectations. The large fleet was a steady drain. British foreign policy continued to be to discourage the appearance of any dominant force in Europe, a view that was sure to bring them into diplomatic if not military conflict with the Kaisers Germany. In 1839, Britain had signed a document (the famous "Scrap of Paper") guaranteeing the independence of Belgium, & it was this that brought Britain into the war when Germany marched in in 1914.
France had been humiliated in 1870. It was plain that it was no longer the dominant country in Western Europe, & was living on it's past. In 1871, France had lost Alsace & Lorraine to Germany, & dearly wanted this territory back. To stave off future German aggression, France allied itself to Czarist Russia, thus threatening Germany with encirclement fuelling German paranoia.
Russia itself was in turmoil, already the seeds of the coming Revolution had been sown, despite belated reforms (eg the end of serfdom). While Russia was strong in population, it was backward industrially. Many of the European nations did not totally trust Russia, as they feared Russian expansionist aims southwards.
Austria-Hungary was a broken reed, a loose confederation of states & peoples which was rapidly coming unravelled. Many of these states sought independence from the empire, & it is not surprising that the spark for war came from the Balkans. While it was a Serb who fired the fatal shot @ Sarajevo, it was unreasonable demands from Austria threatening Serbia that ensured the squabble in the Balkans would lead to more wide-ranging war. The two Germanic states were of course, allies, this is what pulled Germany in.
Germany was the new major power in Europe. With an expanding population & industry, Germany, for centuries a loose amalgam of small states, was now united under Prussian leadership. Germany felt itself surrounded by unfriendly, envious nations in Europe. It colonies were an expression of its need to expand somewhere. The futile naval race with Britain reflected it's growing industrial might. German grand strategy for a major European war was to hold on the Eastern front & strike rapidly against France (the Schlieffen Plan, which dated back at least 20 years). With France destroyed the Germans would be free to turn on Russia at leisure. German strategists discounted British interference. The British Army was considered too small to affect their plans, & anyway the British performance against the Boers (who many Germans championed) was not inspiring. The Schlieffen Plan ws not really relevant to a conflict originating in the Balkans, but when war came in 1914, Germany found that it was the only plan they had on the shelf, so they used it.
In European terms, America at this time had little interest. The USA was inward looking, it had a large land mass to administer & colonise internally, it had little real need of outward expansion. The frontier had only been closed for a decade or so. While America took a close interest in Mexican & Central & South American affairs, Europe was too remote, although it would soon provide a market for growing American industry.
It can be seen that in this volatile atmosphere, the spark in the Balkans was fatal. The military men of Europe had not heeded the warniings from the previous century, & the recent Russo-Japanese conflict. The French doctrine was firmly grounded in attack, preferably with the bayonet. Britain had at least trained its riflemen well, but all the nations underestimated the effect of quick-firing guns & machine guns. Germany had a lot of nice new equipment to try out. I don't know if this necessarily makes them the guilty party though.
