Final answer:
The two major wars fought without a declaration of war are the Franco-Prussian War (1870-1871) and the War of 1812.
Step-by-step explanation:
Two major wars were fought without a declaration of war: the Franco-Prussian War (1870-1871) and the War of 1812. The Franco-Prussian War started after a dispute over the Spanish throne, and although the fighting was localized, it resulted in major changes, including the unification of the German Empire.
The War of 1812, often called the Second War of American Independence, was caused by unresolved issues between the United States and Great Britain, such as the British impressment of American sailors and the support of Native Americans against U.S. expansion. These wars did not require the mobilization of a country's full resources or involve numerous countries.