I have a background neither in history nor the military, so I am ill-equipped to do more than offer a couple of sentences that might give a sketch to readers even less familiar than I about the history of WW1.
For a rigorous blow-by-blow history of the events leading up to WW1, as well as information about the battles and the countries fighting around the world, please see (1) John Keegan (1998), The First World War, published by Vintage Books. He states that the Second World War, was unquestionably the outcome of the First. And the legacy does not stop there. (2) Bible and Sword by Barbara Tuchman (1956), focuses on WW1, but she also identifies problems growing out it; for example, "The Balfour Declaration" in 1917. A study of these links yields deeper insight into present world conflicts. Problems in the Middle East, for instance, spring in part from the British entry into Jerusalem and Palestine in the course of the war with Turkey. While oversimplified, this can be seen as a source of the present conflict over lands sacred to Jewish, Christian, and Moslem religions. (3)The Guns of August, a Pulitzer Prize book also by Barbara Tuchman, (1962), is an easier read on the subject. It has been described as a work that raises history to the level of literature. Shortly after it was published, President Kennedy gave a copy of the book to Prime Minister Macmillan, "observing that somehow contemporary statesmen must avoid the pitfalls that led to August 1914." Maybe it should be a handbook for all presidents, especially today.
The second half of the nineteenth century found a world in which Britain, Spain, France, and Russia possessed large empires, but Germany had almost none. Aggressive acquisition (buying and invading) in Africa, the Pacific, and China (Tsingtao), and two wars with France (1866 and 1871) brought Germany much more land. They were on a roll and wanted a further piece of France.
In overly simplistic terms, WW1 began on August 1, 1914. It was fought between the Central Powers (Germany, Austria, Turkey) and the Allies: the British Empire (population 400 million, including Canada, Australia, and India), the French Empire (population 96 million), the Russia Empire, Italy (May 1915), Japan, America (March 1917), and the colonies of the latter powers. It was over on November 11, 1918 (now Armistice Day). The lives lost was enormous: over 1 million from the British Empire; 1.7 million from France; 1.5 million from Austria; 2 million Germans; 460 thousand Italians; 1.7 million Russians; many hundred thousands from Belgium; certainly more from Turkey (never counted). That is not considering the maimed and seriously wounded. Most importantly, it does not does take into account the much larger loss due to psychological trauma and hatred carried forward to WWII (Keegan states 5 times the loss in life) and through to today in the Middle East.
Germany's plan was to march quickly through Belgium and attack and capture Paris from the north, before the Allies could react. It needed to be done in 45 days. The designer of this plan, Schlieffen, knew that if not completed in the planned time, the war would bog down and Germany would ultimately fail. However. Schlieffen died several years before 1914, and Germany went to war with a flawed plan and without an exit strategy if all did not go well.
Belgium wanted to remain neutral, but refused to let Germany through. For this, they were slaughtered and joined the Allies, thus slowing the Germans down. Further, the French were prepared to resist, having lost precious lands to Germany in 1866 and 1870. Both the peoples of Germany and France were confident that they would win the war in a few months; both celebrated when their troops went marching off to war.
The French were able to stop the German advance before they reached Paris, even pushing them back a bit. But the Germans dug in, making trenches from which to defend themselves, forming a line all the way from the North Sea of western Belgium (25 miles north of Ypres) to the south-east at Switzerland. This was to be called the Western Front. The trench technique allowed the Germans to use many fewer soldiers than in field battles, a reduction in numbers needed because Germany found itself forced to send troops to its eastern territories, where it now was being challenged by Italians and Russians, the Eastern Front.
Deciding it was in their best interest, Britain had signed an agreement with the French to be an ally in case of attack. The British troops came over in larger and larger numbers, initially helping near the north and later helping all across the trench line.
President Woodrow Wilson said America was "too proud to fight". His notion was that plain dealing between nations in open diplomacy was the secret (no pun intended) of evading conflict. However, the Germans tried to bring in Mexico on their side, promising the return of Texas, Arizona, and New Mexico if America went to war against Germany. America declared war on Germany on 6 April, 1917.