One of the many causes of WW1 was the two alliance systems formed at the end of the 19th century and beginning of the 20th. On one side was Germany, Austria-Hungary, and Italy (Triple Alliance known as the Central Powers) and France, Britain and Russia (Triple Entente known as the Entente Powers) on the other side.
For Germany, its newly united states were becoming more and more powerful by the day, enriching themselves in wealth from industries that were overtaking those of Britain. Kaiser Wilhelm II wanted a strong navy that would again rival Britain's, whose was the largest in the world at the time. The kaiser was keen to make Germany the leading country of Europe, expanding his empire and colonising parts of Africa.
Allied with Germany, Austria-Hungary's empire was unstable. Many peoples and cultures (Germans, Austrians, Hungarians, Czechs, Slovaks, Serbs, Italians, Croats and Poles) were all being ruled by the Emperor Franz Josef who was having troubles ruling this vast place. He wanted to colonise countries of the Balkans.
Italy, being the weakest of all Central Powers and with very little industry, wanted to colonise and extend its empire.
France wanted to defeat Germany, take revenge on the humiliation of its loss in the Franco-Prussian war of 1870. In desperate need of allies, it signed an alliance treaty with Russia once the kaiser did not keep up this Russian alliance.
Russia, although rich in area and in population, had few industries, most Russians lived as peasants; the Tsar being in complete control of the country.
Britain was doing well, trading industrial goods all over the world, owning the largest merchant fleet and navy in the world, having an empire stretched across over a quarter of the land's surface. But at the beginning of the 20th century, Britain was being challenged by the USA, France, and especially Germany. Feeling threatened, they started looking for allies even though they had, until then, avoided all European alliances. Against all odds, King George V of England joined the Entente forces against his cousin Wilhelm.
With the most powerful countries of Europe all grouped into two armed forces, tensions and rivalries were rising everywhere.
The alliances settled, Britain and Germany started making weapons as fast as they could. In 1906, Britain made the HMS Dreadnought, the largest, fastest and most dangerous battleship in the world. Soon after, the kaiser ordered 4 Dreadnoughts to be built in the year. Later, Britain announced they wanted 8 Dreadnoughts, built in less than a year. This famous arms race made was loads more likely because after all the efforts made into making these destructive weapons, everyone was expecting and wanting to use them in battle.
Britain and France decided to control the Mediterranean between them, which meant controlling parts of Africa, such as Morocco. Being envious of the wealth and empires of these two countries, the kaiser set out twice to Morocco to try to break the alliance, the second time being a military provocation. But it turned out that the alliance was reinforced and became stronger when one country would back up the other.
When Greece, Serbia, Bulgaria, and Romania gained their independence, they looked to Russia for help when Austria-Hungary threatened them. Russia wanted to secure the Dardanelles so it supported the Balkan states. Serbia had planned to create a Yugoslavia to welcome the southern Slavs of Austria-Hungary. Austria-Hungary then took over Bosnia in 1908. In reaction to this, several "Black Hand" societies rised up in Serbia, devoted to throwing the Austrians out of the Balkan states.
When Gavrilo Princip, a 19-year-old member of the Black Hand organisation, assassinated Ferdinand, Austria-Hungary sent a heavy ultimatum out to Serbia with 10 demands. Serbia declined the 6th one, making Austria-Hungary declare war on Serbia on the 28th of July 1914 after it found out that Russian troops had started to mobilise. From this point, every country in Europe gradually fell into war.
Germany declared war on Russia on the 1st of August then on France on the 3rd of August. Britain declared war on Germany on the 4th of August when it invaded Belgium.
Conclusion: Having said all of this, I think every country is responsible in one way or another but Austria-Hungary more. Even though many trigger events passed, none was the start of the war and no country declared war but the assassination of Ferdinand was apparently enough for one country to provoke a world war and that was Austria-Hungary. Instead of sending an ultimatum in the first place, they could have just killed the murderer. Plus the only reason they sent it was so that Serbia would reject it, giving Austria the excuse to declare war. They started the whole domino effect and even though Germany was responsible for the amount of deaths during the war, Austria-Hungary was at the start of it all.