The Congress was highly successful in achieving its goal, as there were no wars between European countries for almost 40 years. One could say that the Concert of Europe is still continuous, but with different players, goals and name. Thus The Concert of Vienna was first of its kind and in addition Europe had not seen such cooperation between major powers before, it nonetheless made numerous decisions that shaped Europe to its historic course. For instance France was deprived from lands Napoleon had conquered. Britain got several strategic colonial territories, also gaining control of the seas.
France and Spain were restored under rightful bloodlines. However the numerous decisions and outcomes were pompous, concentrating on the needs and wants of four main powers Austria, Russia, Prussia, and Great Britain. The Concerts main accomplishment was the securing of independence for Greece (ironic) by sending fleets to fight against invading Ottoman-Egyptian armies. Thanks to three great powers, Russia, The United Kingdom and France, Greece was finally recognized as an independent nation that it is today. In Belgian revolution the Great Powers recognized Belgium as an independent state. After that the signs of collapse of the alliance started to appear. The goal of the Concert of Europe was to keep a balance of power between leading countries in Europe, but if the balance starts to change to someones favour then the conflicts begin to arise.
The sudden development of Egypt and their successful war against Ottomans brought about fear in European powers. A collapse of Ottoman Empire could have had a destabilizing effect to every country especially to the United Kingdom. England was not only worried for loosing economic benefits and militarily strong state in Mediterranean, but also about Egypts close ties to France. Britain preferred a weakened but intact Ottoman Empire that would give it the strategic and commercial advantages it needed to maintain its influence in the region.
This all lead to what was called the Oriental Crisis of 1840 where United Kingdom, Austrian Empire, The Kingdom of Prussia and the Russian Empire backed Ottoman Empire against Egypt. France did not accept with the terms The Concert of Europe offered to Egypt, not siding with any of the rivals, but stayed neutral. Eventually after numerous military conflicts Egypt accepted the terms and Ottoman Empire got back all its lost fleets and territories. The significant downfall of The Concert of Europe was Crimean War in 1853 when Russian Empire started its expansion, which was against the main idea of the Congress of Vienna. It was all powered by the weakening Ottoman Empire and religious disputes till Russia made its move on Constantinople which was owned by the Turks.
Afterwards European powers joined in to prevent Russia from growing too powerful. Followed three years of hostilities on five theatres till Russian Empire was eventually stopped. Being the first major armed conflict in Europe after the settlement at the Congress of Vienna, Crimean War signified the downfall of the Concert of Europe. At the Treaty of Paris, signed on March 30, 1856, the real looser was Austria. Having chosen to defy the Russians in the Balkans, Austria lost main ally, and over the next few years it appeared that Britain and France were not interesting in cooperation.
Moreover, the Crimean War was the collapse of the Vienna Settlement, the system that had enabled Austria, Britain, France, Prussia and Russia to cooperate and maintain peace for three decades. The Crimean War placed the basis for two powerful new nation states Italy and Germany. The new six-power European system demonstrated to be less stable than its forefather, while the expectation that political and diplomatic aims could be satisfied by war led these states to adopt ever closer alliances. The Crimean War was a turning point in European history, marking the end of the Vienna settlement, and the beginning of a new system. In spite of all, the Great Powers finally returned to war in 1914, almost hundred years after the Congress of Vienna. Alliances and common pacts all end in the course of time, lead by human needs and personal gains, sooner or later. (And the outcome is war ofc)
The First World War begun with a simple murder of Austrian-Hungarian royalty, at least that is the simplified reason. In reality it only triggered the war; the actual roots of the war are much deeper and more complicated than a simple bullet in the Archdukes neck. Rise of nationalism and its movements in Europe created such secret societies that used terrorist methods to promote their views. Slavic people in Bosnia and Herzegovina wanted no longer to be a part of Austria-Hungary, but instead be to be a part of Serbia.
Viewing it in this way, it could be said that nationalism led Slavic people to rebel against their occupiers that ended with the First World War Increased military and naval rivalry led not only to the belief that war was close and increase in military control of the civilian government also there was an increased cooperation between allied countries. For example United Kingdom, France and Russia held secret military talks. The British and the French naval authorities agreed that the French navy should be concentrated in the Mediterranean and the British in the North Sea. Germany and Austria also had military agreements.
When the First World War began, it was to be fought by all powers because they were locked into the conflict from the beginning by contracts and alliances. European powers had formed themselves into two rival groups called the Triple Entente versus the Triple Alliance and the interests of these groups clashed in many parts of the world before 1914. Whenever a clash arose, the two groups seemed to be on the edge of war. Believably, on some level, it eventually led to hatred of each other.
Contributing fact was that after 1870, the European nations began to acquire colonies in Asia, Africa and the Pacific. Their imperialistic activities led to another clash of interests. It could be said that the underlying causes for war are the same as for everything else in life. One decision affects the other to ad nauseam. It was possible that politicians knew that war was at their doorstep, but I am sure they did not know that over 15 million people will have to pay for it with their lives.
Could the First World War be avoided is doubtful, because of the tense relations, rivalry and complicated alliance systems. It was a small scale war between Austria-Hungary and Serbia that transformed into world war. The same could easily happen today when China or USA supports North or South Korea over the conflict. History repeats itself and we need to learn from it. Poking your nose into other peoples business is not the smartest plan when both sides have powerful brothers positioned farther back.