Austria felt that Serbia was overreaching its territorial boundaries and that the only way to forestall it was a preventive war. The death of Ferdinand provided a convenient excuse to go to war with Serbia. In support of Serbia, Russia mobilized its forces against Austria-Hungary, who had declared war on Serbia. Germany declared war on Russia. The United Kingdom and France, allies of Russia, also entered the conflict.
A web of alliances existed between many European countries. Austria allied with Germany, and Serbia allied with Russia. The alliances obligated each country to go to war on behalf of its allies. This dragged more and more countries into the war. The war pitted the Central Powers, consisting of Germany, Austria-Hungary, the Ottoman Empire and Bulgaria, against the Allied Powers, consisting of the United Kingdom, France, Russia and Italy. The United States ultimately entered the war on the Allied side.
Lincoln hoped to use a well-known figure of speech to help rouse the people to recognition of the magnitude of the ongoing debates over the legality of slavery. His use of this paraphrased metaphor is perhaps clearer when you look at some more of his speech:
"A house divided against itself cannot stand." I believe the government cannot endure permanently half slave and half free. I do not expect the Union to be dissolved — I do not expect the house to fall — but I do expect it will cease to be divided. It will become all one thing or all the other. Either the opponents of slavery will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or its advocates will push it forward till it shall become alike lawful in all the States, old as well as new — North as well as South.
As you can see, in this metaphor, the "house" refers to the Union — to the United States of America — and that house was divided between the opponents and advocates of slavery. Lincoln felt that the ideals of freedom for all and the institution of slavery could not coexist — morally, socially, or legally — under one nation. Slavery must ultimately be universally accepted or universally denied.
The First Continental Congress organized a boycott of British goods. The Second Continental Congress declared independence from Britain.
