A year ago at this time, there was a mood of near-euphoria about the global economy: A boom appeared to be still under way in the United States and was expected to boost activity everywhere else. Moreover, the worst of the Third World debt crisis seemed comfortably behind.

President Reagan, in a highly optimistic pre-election speech to the annual meeting of the World Bank and International Monetary Fund here, touted the American "locomotive" role, inviting other countries to continue to export their goods to eager American buyers. He promised that the American "blessings of progress" would be shared with others to achieve "golden dreams for all mankind."

With the benefit of hindsight, we now know that the United States actually had begun to enter a "growth recession" at mid-1984, and that the warnings offered by others -- especially the long-term dangers of an overvalued dollar and a huge budget deficit -- deserved more attention.

Today, the rose-colored glasses can be put aside. A slump in the United States, caused mostly by the havoc wreaked on American exports by an overvalued dollar, dropped the growth of real gross national product to 1 percent in the first half of 1985. Only Reagan party-liners see a solid recovery in the second half. Others think we'll be lucky to move up to a 3 percent growth rate.

Europe, still beset by high unemployment and playing catch-up ball with the United States and Japan in the critical area of high-technology production, is expected to show a real growth rate of not much better than 2.5 percent any time in the near future.

And as an undercurrent shaking confidence on a global level, there is the real specter of a protectionist backlash in the United States, a gut reaction to mounting trade and current account deficits that have transformed this country from creditor to debtor status.

Protectionist measures would curb the volume of world trade. This poses a tremendous threat to the Third World debtor countries such as Mexico that have come a long way back from the depths of the crisis in 1982, but desperately need greater economic growth to survive and be able to pay off their debts.

Compounding the problem is the reality that the commercial banks don't want to make new loans to Third World borrowers and that no governmental agencies are coming forward with more generous official money.

In remarks to New York bankers the other day, Jesus Silva Herzog, the highly regarded Mexican secretary of finance, warned soberly that prospective economic growth rates in First World just aren't good enough to pull the developing countries out of their rut, and suggested there is "a creeping sense of complacency" among the rich nations.

"We cannot continue to request sacrifices from the Mexican people without offering in exchange a better future for them," he said.

"We are talking about the potentially most explosive development issue of our times. In industrial countries, it may be seen as a purely financial or economic issue. In debtor countries, it is seen as a serious political and social issue. When real income is falling or jobs are lost while real transfer of resources are negative, debt becomes a critical matter in which long-term gains are not necessarily balanced with short-term sacrifices."

The link between the prosperity of the rich industrial nations and the health of the developing nations -- both advanced and primitive -- is increasingly recognized by analysts. John C. Sewell, president of a Washington think tank called the Overseas Development Council, points out that the United States buys one-third of all manufactured goods turned out by the Third World.

On the other side of the ledger, the developing countries had been buying about 40 percent of American exports. A major reason for the $123 billion U.S. deficit last year was the inability of Mexico, Brazil and other smaller countries to buy as much from American manufacturers as they used to.

"If their debts are to be repaid, trade liberalization is at the core of U.S. interests," Sewell warns.

But the signs are pointing the wrong way: Despite the Reagan administration's belated attempts to dilute the protectionist fever on Capitol Hill, it looks as if some serious restrictive measures, starting with tougher textile quotas, are likely to be put into effect this year. If retaliation is the response of America's trading partners, a shrinkage of world trade could result, triggering debt defaults by some important Third World borrowers.

Federal Reserve Board Chairman Paul A. Volcker has cited another risky consequence of protectionism, casually overlooked by those senators who say that "we must do something" to counter the success of the Japanese in penetrating the American market: Hostile action to cut down Japanese exports could cause an exodus of Japanese capital from the United States.

Much of the profits earned by Japan from its sales of cars, electronics and other goods is recycled here in the form of investments in Treasury notes and bonds, helping to finance the U.S. budget deficit. If Japanese capital is pulled out, the Fed would have to raise interest rates to attract enough domestic capital to replace it.

"If you begin fooling around like that, you don't know what the reaction may be," Volcker told the House Banking Committee.

Is there a silver lining among all these clouds? If the dollar continues a gentle decline, and lower interest rates stimulate a solid late-1985-1986 U.S. recovery, some analysts believe that American exports might be rejuvenated, allowing the trade deficit to begin receding some time in 1986 and defusing the protectionist trend.

But this would require further substantial cuts in the American budget deficit, a more generous financial helping hand for the Third World, and European-Japanese willingness to expand their economies to absorb more American goods. This all calls for global economic leadership -- and no one so far has stepped forward to assert that responsibility.