Final answer:
The Dual Alliance was a defense alliance formed between Germany and Austria-Hungary in 1879, aimed at protecting each other. It was a significant part of the larger system of defense alliances that contributed to the start of World War I.
Step-by-step explanation:
The Dual Alliance, described by Langer, was a defense alliance formed between Germany and Austria-Hungary in 1879. This alliance was aimed at protecting each other in the event of an attack. The Dual Alliance was a significant step in the complicated system of defense alliances that eventually led to the start of World War I.