Britain. Here is why, and the background evidence.
Britain, was on a downslide after the Franco-Prussian War in 1870-1. After Prussia and the German Confederation trounced the French, the industrial strength shifted totally to the newly founded German Empire. Prussia, Bavaria, Wurttemburg, Baden, and Hesse; were all "city-states" at best, compared to the mammoth French, Russian, British, Austrian, Ottoman, and emerging Italian nations. And all of them had something in common and something to bring to the new empire to throw the balance of power decisively in their gain. With the given "city-states" you had the: best land army in Europe, the most technological army in Europe, the 3rd richest region in Europe, 5 major rivers, vast amounts of coal, iron, and people, and further more, the most educated society as-a-whole. When they came together, allied after a brief scuffle with Austria-Hungary, a temporary alliance with Russia, an alliance with Italy, and an alliance with the Ottoman Turks, Britain was out-numbered, out-manned, out-politiced, out-manuvered, and soon being out-produced. In just 30 years, the new German Empire exceeded Britian in finished products, military size, imports, exports, and controlled over 60% of Europes railroads and was looking to expand into Asia and the Middle East! Britain couldn't stop them. So what did they do? Ally with their bitter enemy the French to save face and play the even more ancient animosity between the French and Germans. They colluded with the Russians, in their 200 year on-going attempt to modernize, by convincing them to drop the Germans as their ally and pick up them and France as their new buddy. You see Germany wanted to build and give the technology to Russia to help them modernize, but Britain and France offered cash and assistance, which is what Russia wanted more for two reasons, one cash is easier to misappropriate (or find its way to the pockets of royalty and admins) and also it forced Russia to build the new Russia and actually learn the technologies and not just use it.
After that Germany and the unstable Austro-Hungarian Empires felt shafted again by Europes "Old Elite." To now get to Asia and the Middle East, the Germans and Austro-Hungarians had no option but to build towards Istanbul, rather than Moscow and Kiev, to expand their trade system. Britain had the seas and oceans hook-line-and-sinker, with the biggest, baddest, most technological, and fastest navy on Earth, with a presence in every ocean and sea. Britains problem was, for 200 years they were the only industrial power with the capablity to send their products abroad, so naturally they TOLD people what they could buy, and didn't ASK people what they wanted. Germany, took advantage of this and was starting to outpace Britain WITHOUT EVEN HAVING THE RAILROAD TO ASIA OR A NAVY TO GET IT TO AMERICA!! PANIC PANIC!!
There was the background, now my case.
After this Germany started building up their navy, at an alarming rate, (because the railroad was being blocked and stalwarted by the Brits and unstable Ottomans and Austro's at every turn) so too did Austria-Hungary and somewhat the Ottomans and Italians build up their navies as well. The British policy of having a navy bigger than the next 3 navies was becoming impossible. So the audacious British told Germany, Italy and the rest; "Your Navy is a luxury, we need ours for our huge Empire, you don't..." Talk about balls.. Well none of them bought it, so then Britain turned to France and Russia and colluded to block further German attempts to build a railroad to Asia and the Middle East, again pissing off the Germans and Austro's. Then when Germany opened the Keil Canal, the British used neutral Belgium as an excuse to send a larger force some 300 miles from the canal (technically in neutral water which is a BIG NO NO) to "help Belgian neutrality in the face of an aggresive force." Then Britain cuts the International Cable that went from mainland Europe to the US, Canada, Mexico, etc; and sends their "version" of the happenings in Europe to the US, etc. Germany and the Austro's feel like and are being treated like "the little brothers of Europe" when in fact "they are all grown-up and just as strong and smart as the 'Old Elites' (Britian and France.)"
As you can see, things are already getting hot, and Britain is about to make 7 BIG policy mistakes and miscalculations, that throw gasoline on the "Spark lit in Sarajevo" or Ferdinands assasintation.
1) They compel Russia to be, as Russia claimed always to be, "the Protector of the Slavs" meaning Serbia and the rest. (Princip the killer of Ferd, was a Serb Nationalist) The problem is Russia is truly a bear. Takes forever to wake up, but when it does, you can't stop it and it wrecklessly swings at anything in its path.
2) Play on France's desire to regain Alsace-Lorraine. Part of the settlement from the Franco-Prussian War was Germany got 2 sizable states from France as a result of victory, and France bitterly wanted it back. Britain promised "If ever there came a struggle, remember Alsace-Lorraine, and you have it, plus whatever intrest you wish to compound!" This was something deeper and more passionate in the French psyche than the British thought or cared.
3) They promised Russia back the land they lost by the humliating defeat to the Japanese 10 years earlier, mainly Manchuria and parts of modern-day North Korea. And with the prodding from the French offer the rest of Poland as another prize if stuff gets heated. Germany takes note of this and now looks to punish Russia for the notion.
4) Independence to Middle Eastern nations if: "...the event should arise in conflict between the Ottomans and Britain, you will be freed by Britain from the clutches of the Ottomans..."
5) Assured both Russia and France, as well as their own British subjects, that "....if war happened it would be quick and decisive..." and "...after war everything is better..."
6) Sent a non-German speaking diplomat to Germany to convey the attitude and resolve of Britain and her allies of Germany's seeming aggressive tactics and "reckless arms escilation." The Germans took this as both an act of arrogance, weakness, and general stupidity, which further envigorated the war happy officials that France and Russia were weak, and Britain is desperate
7) Move their naval fleets closer to the German, Austro, and Ottoman nations and colonies, and further excite an already itchy trigger finger and blatantly intense situations in the North, Medditeranain Seas, and Atlantic Ocean.
Then, when Ferdinand is killed, Russia, knowing its own limitations, immediately starts mobilizing, the Austro's having already mobilized, sees this as a "blink" so it attacks and occupies Serbia, after negotiaitons go sour between the Serbs and Austro's. The Germans, who seemed to be pushing the Austro's along, see the Russian deployment as an act of aggression, so they mobilize and strike first, only because Russia is moving slower than anticipated. Now Britain and France are looking at each other like "Oh ****..."
So then Germany, knowing both the French and Russians are allies, attacks France via Belgium, which the British people had been told "is a neutral country." The British people, really cared less about the Russians and French, but the Belgians were "innocent bystanders getting mauled by the Hun (Germany)" So now Britain by both alliance and popular obligations is forced in to the fight as well.
If Britain would have been the true capitalist they claimed and are famed to have been, they would have accepted the German competition and sought to out sell them, and easliy could with their mammoth colonial holdings. The British, dug their own holes with their arrogance and elitist view, and became more complacent and dense to what the world was becoming and going. Britain played on a lot of sensitive notions that they really cared less and knew nothing about. At every turn, up to Sarajevo, Britain had the power and prestige to stop or relive the tension, but didn't. Britain was used to machine gunning down natives (as were the rest of the Europeans to be fair) and not fighting similar armed and tactically sound armies and nations. Britain could have let the eventual thing happen, and let the Berlin-Baghdad railroad be built, and focus on the bigger picture, sealiner trading, but wanted both no matter how impossible or stupid it seemed.
I say this, I don't blame all British people, that's silly. The mentality and policy of Britain at the time, especially it's leaders, however antiquated or arrogant they were, was to blame more so than most, but not 100%, maybe 50%.
I put like this in my thesis.
50% British Policy and Leaders
40% German Policy and Leaders
5% French Policy and Leaders
3% Austro-Hungarian Policy and Leaders
1.99% Russian Policy and Leaders
.01% Gavrilo Princip and the Black Hand