For any history buffs who might be interested, today is the 100th anniversary of the assassination which caused World War I. This war, in turn, had drastic effects on Ireland and again led to (first) many Irishmen fighting more wars and (second) many Irishmen leaving Ireland as another generation of Wild Geese.
Whatever your take on Gavrilo Princip the Archduke's assassin, there is no question that by his pulling the trigger of a gun a twice, he changed the course of human history. His actions ignited WWI. This war essentially caused WW2. These wars together caused the "Cold War" which gave us the Korean War, the war in Vietnam, and various military actions all over the world.
WWI brought about the end of the Ottoman Empire and created various Islamic nations, and WW2 brought about the creation of the state of Israel. Needless to say all of this has lead to the deaths of millions of people, created massive human misery, wasted untold human and natural resources and, by contrast, has accomplished very little. And it still goes on.
Some argue that if Princip had not shot the Archduke something else would have set off the war anyway ... and that might be true. I would only point out that our actions (and at times our words) have consequences that for the most part we cannot even imagine.
Most Irishmen did not enter the war until Britain declared war on Germany on August 4, 1914. Then on the 9th of August, the B.E.F. began embarking for France. The first battle took place on August 22, 1914 -- the Battle of Mons.