This will be the most critical year for American-Japan relations in the postwar period, according to the United States ambassador to Japan. Ambassador Mike Mansfield says that significant progress must be made toward opening Japan's markets to imports and reducing its trade deficit with the United States, or else the US will move increasingly toward protectionist trade policies.
And the ambassador fears that unless the year shows positive results in the US favor, the trade imbalance will become an issue in the 1984 presidential campaign.
''This [US-Japanese] relationship is so strong, so valuable, so much depends on it - we handle 30 percent of all international trade - that we just can't afford to let each other down,'' said the ambassador in an interview.
''We're an impetuous people,'' said Mansfield. ''We like to see things done yesterday. The Japanese would like to do them tomorrow, figuratively speaking.''
''On our side, we've got to develop understanding and patience, and do what we can to bring about a continued economic recovery in the US,'' he continued. ''And Japan, on the other hand, has to open its markets more and to liberalize its imports and assume a more responsible position in the international trading picture of which it has been the chief beneficiary. . . .''
He cited, among other things, the so-called domestic content bill passed by the US House of Representatives and a Senate resolution aimed at the import of Japanese machine tools.
''If things don't get better, the prospects for protectionist legislation will increase,'' said Mr. Mansfield.
The ambassador was in Hong Kong along with other American ambassadors and chiefs of missions for a meeting with Secretary of State George Shultz. Shultz said that protectionism had been the subject of a great deal of discussion with the Japanese during his visit to Tokyo last week.
Ambassador Mansfield said of Japan's Prime Minister Yasuhiro Nakasone: ''He's a new type of Japanese leader. He's trying to lead.''
Mansfield said that on trade and defense issues, Mr. Nakasone had to fight not only against opposition parties but also against factions within his own party. He said that Nakasone ''took a big risk'' in rapidly deciding on preliminary steps to liberalize Japan's markets and to strengthen its defenses. These moves may have raised unrealistic expectations among Americans about how fast Nakasone can move in the future, Mansfield said.
''. . . there are certain things that we should do. . . . It's going to take all of us working together, and the mainspring will be the US,'' he said.
On another note, Mansfield expects that within the next few months, US-Japan talks will begin to plan Japanese naval patrolling out to a distance of 1,000 miles from Japan. This would help protect sea lanes and take some of the pressure off the US Seventh Fleet.