In my opinion the most important alliance in the lead up to WW1 was the Austria-Hungary and Germany alliance. Germany gave full support to A-H in case of war in July. In my opinion this encouraged A-H to become a lot more aggressive, after this alliance was formed, they knew that they could do anything with the support of Germany behind them; this led to their aggression in the Balkans.
A very important alliance was between Russia and Serbia, in this alliance Russia promised Serbia that they would defend it from attack and retaliate against the country who attacks Serbia. However at the time Austria-Hungary had control over parts of Serbia and parts of the Balkans, this conflicted with the Russians policy and therefore provoked attack.
There was the Dual Alliance of 1879 between Germany and Austria-Hungary, the Triple Alliance between Germany and Austria-Hungary and Italy of 1882, the Franco-Russian alliance of 1891-4, the Entente Cordiale between France and Britain of 1904 and the Triple Entente between France, Britain and Russia of 1907.
From those alliances one can clearly see that Germany, Austria-Hungary and Italy were on the one site, and Britain, France and Russia on the other site. Even though the Triple Alliance broke, and the Triple Entente just settled colonial disputes, especially the Dual alliance as well as the Franco-Russian alliance played a major role for the outbreak of the First World War. Thus, the Alliance System was very much responsible for the First World War.
The Dual Alliance between Germany and Austria-Hungary was formed in 1978. The agreement states, that if either of the countries is attacked by Russia, the other will come to its aid with all forces, and if attacked by another country, it will at least stay neutral. This is very significant for the outbreak of the First World War as Austria-Hungary sought Germany’s agreement for its declaration of war on Serbia, and further Germanys support in case Russia would attack Austria-Hungary as result of the war with Serbia. Germany promised its support to Austria-Hungary (known as the ‘blank cheque’) and as a result of Austria-Hungary’s declaration of war on Serbia and Russia mobilized. Germany promising support to its alliance Austria-Hungary as a direct consequences led to war. If Germany had not promised support, Austria-Hungary would not have been able to declare war on Serbia, as it would not have risked a war with Serbia and Russia.
A second, very important alliance was the Franco-Russian alliance, which was to remain effective as long as the Triple Alliance was renewed. Thus, the alliance between France and Russia was more or less due to the alliance between Germany, Austria-Hungary and Italy. In the Franco-Russian alliance France and Russia agreed to provide troops in case of an attack of Germany, Italy or Austria-Hungary. Further, it stated that, if one or more members of the Triple alliance mobilized, then both, France and Russia, also should mobilize.
Germany knew about this alliance, however, not the exact terms. Further it was known that France’s railway and communication systems were better than that of Russia, thus France would be able to mobilize faster than Russia. As a result, Germany developed the Schlieffen Plan in about 1882, which would prevent a two-front war, as the idea was to quickly take over France through Belgium and than to turn to Russia, which, according to the Germany military leaders, by that time, would just have finished mobilizing.
Therefore, when Russia mobilized in 1914, Germany knew that if it would declare war on Russia only, France would get involved due to the Franco-Russian alliance. Since Germany did not want to risk a two-front war, it immediately implemented the Schlieffen Plan and declared war on both, Russia and France. Furthermore the Schlieffen Plan violated Belgium’s neutrality, as France was to be taken over through Belgium. Therefore Britain got involved, as it had guaranteed the sanctity of Belgium’s borders.
In conclusion, even though some alliances were weak and ineffective like the Triple Entente, others like the Dual Alliances were more effective. Furthermore the alliances certainly gave Austria-Hungary and Germany, as well as Russia, confidence to threaten and finally go to war. The Alliance System also created military plans like the German Schlieffen Plan, which was developed after the Franco-Russian alliance was signed. The Alliance System was certainly not the only cause of the First World War; there were many others, which altogether led to the outbreak. However, when one sums up all the events, which were due to the Alliance System, I can conclude that the Alliance System was one of the major causes of the First World War.
Here's what a teacher thought of this essay
There are some good points in this answer but overall it lacks depth and range of factors. It is always important in History to offer a balanced analysis - to consider all possible reasons for an event and then weigh up their relative importance before reaching a conclusion.