World War I begins with Germany's invasion of Luxembourg. The same day, Germany and Russia declare war against each other, and France mobilizes. At the end of July, Europe's tenuous peace collapsed when Austria-Hungary declared war on Serbia. By August 5, Russia, France, Belgium, Great Britain, and Serbia had all lined up against Austria-Hungary and Germany. One month later, the bloody Battle of the Marne was fought outside Paris, halting Germany's advance to the sea and effectively deciding the future course of the war. World War I would be a terrible war of attrition, won by whichever side was willing to commit the greater number of men and resources to the lethal trenches.

And I would add that, IMHO WWI was the start of WWII. Many historians
feel this way and have written books on the topic. And what is also
mentioned is the situation of Woodrow Wilson's health, his stubborness
about the League of Nations and the fact that his wife withheld from the
American public the true condition of her husband's physical condition.
Some historians — and in particular Phyllis Lee Levin in her book,
"Edith and Woodrow" indicate that the the first lady ran the country from
the white house for over a year, until the President's death, but all the
while giving the impression that Wilson was capable and aware of the
affairs of state. Just an astounding book.

I would also recommend Barbara Tuckman's book "The Guns of August"
which is a fasinating book on what lead up to WWI and the first four
months of the war, and when the Allieds stopped the German army before
Paris as Jan mentioned. I believe of the two Pulitizer Prizes that Tuckman
won, that "The Guns of August" was a Pulitizer Prize winner. Her book on
Vinegar Joe Stillwell written, earlier, won the Prize as well. All of the above
is my OPINION, you understand.