Final answer:
China became the topmost trade problem for the U.S. at the beginning of the 21st century, due to a significant trade deficit caused by high volumes of imports from China and comparatively lower exports to China. This situation became more concerning in light of the U.S.'s historically less favorable attitude towards globalization and expanding trade.
Step-by-step explanation:
The trading partner of the U.S. that became its topmost trade problem at the beginning of the 21st century is China. Throughout the end of the 20th century and into the 21st, there were persistent concerns about the United States' growing trade deficit with China. This became a significant issue as China's economy expanded rapidly, partly due to globalization, which led to an imbalance in trade. The U.S. saw a large volume of imports from China while exporting comparatively less, causing a trade deficit. This situation contrasted with the U.S.'s trade relationship with other partners such as Canada, Mexico, and Japan, which did not pose the same level of concern regarding trade imbalances.
Historically, issues with trade deficits had previously been voiced in the context of the U.S. and Japan. During the 1970s and 1980s, Japan was seen as a major competitor that was outcompeting U.S. manufacturers, particularly in consumer electronics and automobiles. However, by the beginning of the 21st century, this concern had shifted towards China.
It is also relevant to consider the general attitude of the U.S. towards globalization in this context. Americans have shown less favorable attitudes towards increasing globalization compared to citizens of other countries, including China, where there is a higher level of support for expanding trade ties. This sentiment has been reflected in trade policy and public debate concerning international trade.