Your assertion is spectacularly incorrect. Long story short, the trenches were built through Flanders, northern France and Alsace after the German attempt to take Paris failed. Neither the French nor the Germans had any intention to cede the initiative to the other; both had offensive plans, Germany's was just relatively more successful. The German mobilization plan actually included an attack on France through Belgium and Luxembourg, making Germany the only European country which could not mobilize without going to war.

There will undoubtedly be a lot of stunning misconceptions flying around this thread in general as people come in to help you out, and one of our flaired users who specializes in WWI wrote an absolutely fantastic article in which he goes out of his way to explain the evolution from the era of maneuver into the era of stagnation, that is, Trench Warfare.

The long and short of it is, however, what /u/Serpenz and /u/thundersaurus_sex (nice handle, by the way) have already touched upon. Both sides had relatively matured offensive strategies for dealing with the other, all of which find roots immediately pre- and post- conflict in the 1870s. The fortifications, especially on the entente side, were always of a necessary, temporary nature - the attack was always to be resumed. To say either side 'relied' on it as if it was some pre-born strategy is a highly incorrect use of phrasing. The German's success is largely attributed to a more mature infantry fighting doctrine and a blithe willingness to disregard the neutrality of states, such as Belgium, in order to achieve strategic envelopment. The Schifflien plan worked so far as that was concerned, but the French pulled back into a defensive line near Paris, rather than risk (foolishly) a decisive battle that would've been fought very much on the German's terms.

Finally, its always good to remember that WWI was not just the Western Front, whose static nature is wholly anomalous. The other Fronts never lost their relatively high tempo of movement and operations.

In order to answer this question, we need to break apart the how and the why.

How did trench warfare start or, conversely, why did maneuver warfare end on the Western Front?

Well, to begin, maneuver warfare requires significant planning and resources in both material and human terms. Germany, like all of the major powers, had exceptionally detailed invasion plans that laid out troop time tables, routes of march, railroad schedules, etc. These plans, generally referred to as the Schlieffen Plan, were carried out when Germany invaded France. Likewise, the French initiated their abortive Plan XVII. The German army suffered unexpected set-backs in Belgium and encountered significant resistance in their advance across the Frontiers.

This was a period prior to large scale military mechanization. German soldiers were required to march over great distances and also fight. Several authors (Tuchman, Hastings, Keegan, Terraine) have noted that German soldiers were reaching the end of their stamina by the Battle of the Marne. Men were foot sore, tired, their boots and other equipment falling apart.

It was at this point that Joffre counterattacked. The first Battle of the Marne, really a series of confused and disconnected battles, put an end to a German attack that was already running out of steam. Von Kluck retreated behind the Aisne and began to prepare for an allied pursuit by digging defensive positions. Both sides needed time to recuperate. After the Allied failure to capture the heights of the Aisne, the so called "Race to the Sea," really an effort on both sides to find flanks to turn, began.

As each side came face to face with stiffening opposition, military leaders recognized that a "break through" could only be achieved by massing men, equipment and shells. All of this required a major logistical effort that necessitated detailed planning. In order to protect and build up an army in preparation for an assault, defensive positions are a requirement. They also, of course, constitute the best means to prevent an opponent from succeeding in his own efforts to "break through."

We can now turn to the second part of your question: why did trench warfare predominate?

Today, the popular perception of trench warfare is that it was exceptionally bloody, violent and produced significant casualties. At times during the Great War this perception accurately reflects reality. Yet the heaviest casualties were typically accrued when attacking forces left their trenches in efforts to maneuver. The Battle of the Frontiers produced exceptionally high casualties (roughly 300,000 for both France and Germany) as both sides found opportunities to bring modern weaponry to bear on troops moving in the open. The same is true of the Hundred Days Offensive in 1918. While trench warfare produced a steady trickle of casualties even in "quiet" areas, any attempts by either side to leave their works almost always resulted in significant casualties, especially in the first years of the war.

Trench warfare became the norm because it was the most effective way to shelter troops from modern, long range weapons. While rifles and machine guns were devastating against troops with no cover, the real killer during the Great War was artillery (Sheffield, Holmes). Trenches also provided a strong defense which, as the years went by, became increasingly complex with various schemes to provide "defense in depth," and "zones of battle." Prior to the development of the early combined arms concepts of 1917-1918 and the debut of armored vehicles, trenches were a powerful form of protection that would not only blunt an enemy attack, but would allow the defenders the ability to use their own weapons to great effect.

As technology advanced and the coordination between infantry, artillery, aircraft and tanks increased, trenches became less and less useful so that by 1918 attacking forces were able to seize defensive positions which would have resulted in bloody failure only a few years before.

Barbara Tuchman. The Guns of August. Max Hastings. Catastrophe 1914. John Keegan. The First World War. John Terraine. The First World War 1914-1918. Richard Holmes. Tommy. Gary Sheffield. Forgotten Victory.

You are right that all parties thought the war would be over soon, think about all the wars fought in Europe up till this point. They usually only lasted a few months and few thousand deaths at worst. So they had literally no idea in the world what to expect or what would happen. On top of that, majority of treaties were kept secret at that time, so when two countries declared war, there was all of a sudden 6 more at war by default.