Was the alliance system the most important reason for increased rivalry between the great powers in the period 1900-1914?
there was rivalry before the alliance system formed. Germany and Britain were the most powerful countries and Britain felt Germany was challenging Britain and the countries began to compete. This quickly turned into rivalry. Most countries in Europe had an enemy at this time and therefore waned a friend to back them up incase war occurred. This is how the Triple Alliance and Triple Entente came about. Rivalry increased from there.
Britain had a small army and because of this had to have a powerful navy. In 1900 Britain had the largest navy in the world. Britain became fearful of Germany as it seemed to be trying to overpower her navy, these fears were partly responsible for the alliance system. Britain's leading naval supremacy was in old ships. Dreadnoughts destroy any older ship.
There became a race to build the most dreadnoughts 'Germany V Britain'. By 1914 Britain had more dreadnoughts then Germany. This wasn't a main cause of Britain going to war. 1909-1910 it was the peak of the race. Britain planned to build 4 but because of pressure from the public the ambitions of 4 were doubled to 8. This race made Britain resent Germany but it built its relationship with France.
Austria Hungary was a country of many nations. On the surface it seemed a very powerful country, it had strong forces and a big empire. - But this country was falling apart and about to crumble. The nations within Austria Hungary were very nationalistic and wanted independence from Austria Hungary or the option to go back to their original country. The assassination of Franz Ferdinand (the archduke of Austria Hungary) was an attempt of the Black Hand Gang, from Serbia, to 'free' all the Serbians from Austria Hungary. The Black Hand Gang was so committed to having all Serbians in Serbia that at least three of them were willing to kill Franz Ferdinand. They would then commit suicide because they were scared of the consequences.