Russia mobilized a vast army against Austria-Hungary in favour of Serbia, one of their allies. And then on August 4th 1914 Germany entered WWI when they invaded Belgium, thus starting WWI. In September, 1915 Germany was putting in a lot of effort so America would not enter WWI. But on April 6th, 1917 the USA enters the war after President Wilson outlined his case to congress about WWI, just four days before.
There are so many more events that took place during world War One, these are just a few. Leading up to USA involvement in the war.
It would be "germs and disease" that had the greatest impact when the Old and New Worlds met, since the Natives in the New World were not immune to the germs that the Europeans were carrying--leading thousands of Natives to be killed.
Europeans needed new sources of raw materials. The European scramble to build empires from the 1870's to about 1910 is often called the? New Imperialism.
