THE MOST EXPENSIVE four words in American diplomatic history were, "I took the Isthmus," spoken almost offhandedly by Theodore Roosevelt in a speech at the University of California at Berkeley in 1911. Because of them, the United States paid an indemnity of $25 million to the Republic of Colombia in 1921, in tacit recognition of a wrong committed. As a nation, we have been paying compound interest ever since.

In this solid, entertainingly written and fair-minded book, David McCullough unravels the complicated and sometimes deliberately obscured story that lies behind the Panama Canal. Seen in the perspective of the present, Panama was a Watergate that worked - an act of burglary in which a strip of land was pried away from its rightful owners, and locked, in perpetuity, in legal control of the United States.

That McCullough excels in imparting life to the desicated facts of engineering was clear in his previous book, The Great Bridge, about the building of the Brooklyn Bridge. Here his task is more complicated, since the story of the Panama Canal is as much concerned with realpolitik as with technology.

Somewhat in the manner of a canal worker manning a giant excavator, McCullough clears away swamps, pierces the jungles, and exposes to light an incredible amount of dirt. The dirt, however, is only a part of his narrative.

As McCullough remind us, the completion of the Panama Canal was the single most costly peacetime project ever mounted. Digging was begun by the French in 1881 and completed by the United States in 1941. Total cost was $639 million in uninflated dollars, or five times and price of all territories purchased by the United States over the span of a century. Some 30,000 canal workers diedin the process, most of them victims of malaria and yellow fever. The amount of earth moved - about 100 million cubic yards - would have formed a tower reaching 16 miles into the sky, with a base the size of a city block.

The feath of construction was matched by advances in public health. Yellow fever was conquered, and malaria contained, by a team of U.S. Army doctors headed by the gifted William C. Gorgas. But in the end, the technological achievement was tainted by the political circumstances in which the Canal Zone became United States territory.

First came the French, inspired by Ferdinand de Lesseps, builder of the Suez Canal, who persuaded thousands of Frenchmen to invest in his Panama Canal company. The difficulties of construction were grossly underestimated, and the company went bankrupt in 1889 in a scandal that came close to tumbling was Third Republic. A new company was formed, which pinned its hopes on the possibility that the United States would buy out the French assets for an asking price of $40 million.

The United States, at the time, was committed to building a canal through Nicaragua. A campaign to reverse that decision was mounted by two agents of the French company, the Wall Street lawyer William Nelson Cromwell, and Philippe Bunau-Varilla, a French army officer who had worked on the failed canal project. Against all odds, the pair succeeded in reversing American policy, and in 1902 Congress denied its endorsement to Nicaragua and favored Panama.

Panama was then part of Colombia, and treaty negotiations began. To the fury of Theodore Roosevelt, the Colombian senate balked at U.S. terms, delaying the canal timetable. Buau-Varilla, who was headquartered in the Waldorf-Astoria Hotel, met with Panamanian revolutionaries and a date was set for an uprising. What assured the success of the Panamanian revolution was the timely appearance of the U.S. gunboat Nashville , whose arrival Bunau-Varilla was able to forecast.

Immediately after the "revolution," Bunau-Varilla - a Frenchman who had not been in Panama for 16 years - rushed to Washington to negotiate, on behalf of the new republic, a treaty that gave the United States rights in perpetuity to the Canal Zone. As Secretary of State John Hay conceded privately, the treaty was "vastly advantageous to the United States," and it is this document that Panama ever since has sought to modify.

The French canal company received $40 million for its assets; Cromwell put in a bill for $800,000 for his services and Bunau-Varilla recovered $450,000 in family investments. "Deniability" had been maintained throughtout - there was not a single compromising document or letter left by Cromwell or Bunau-Varilla. It was only Teddy Roosevelt's frankness, in a 1911 speech, that gave away the show and provided grounds for the Colombian claim for $25 million.

Along with others who gave examined the evidence, McCullough believes that if the United States had waited for another six months, Colombia would have approved the draft treaty, and the canal could have been built without legal taint. But like a future Republican President, Theodore Roosevelt was persuaded that his motives were excellent and was certain that history would vindicate him. In T.R.'s words, "I took the Isthmus, started the canal and thn left Congress not to debate the canal but to debate me."

The debate still continues. Anyone who reads David McCullough's solid, readable and documented account will know what the argument is about.