Modern Germany was created when Bavaria and other small German states merged with Prussia in 1871. A republican government emerged after the monarchy was toppled in 1918.
The creation in 1871 followed a war between Prussia and Austria to determine which of those large German states would dominate the smaller ones. After Prussia won, Austria looked southwards for expansion (resulting in Austria-Hungary, which encompassed much of the Balkans as well). Hitler always felt that Austria should have been included in the new Germany, and made it clear in his book.