With which countries does the U.S. have important mutual defense treaties?

The United States has important mutual defense treaties with South Korea, Japan, and Australia primarily through agreements such as the Mutual Defense Treaty with South Korea, the U.S.-Japan Security Treaty, and the ANZUS security treaty with Australia. These treaties signify a commitment to mutual defense in the event of an attack on either party, reflecting strategic alliances in the Asia-Pacific region.

In contrast, while Germany, France, and Britain are key NATO allies, the nature of their defense agreements differs, focusing more on collective security under the NATO framework rather than specific bilateral mutual defense treaties. The other options involving Brazil, Argentina, and Chile, and South Africa, Egypt, and Nigeria do not involve significant mutual defense treaties with the U.S. that match the prominence and historical commitment found with the nations listed in the correct answer.