This month, with full diplomatic panoply and in the presence of most of the hemisphere's Latin American leaders, President Carter and Chief of Government Omar Torrijos Herrera of Panama signed two treaties which will, if ratified, give control of the Panama Canal to the Republic of Panama by the year 2000, while guaranteeing the canal's neutrality. But ratification by the U.S. Senate, which requires a two-thirds vote, is far from assured. A close observer of the continuing debate is 44-year-old Yale graduate David McCullough, who spent 5½ years working on his current best-seller, The Path Between the Seas: The Creation of the Panama Canal 1870-1914 (Simon and Schuster, $14.95). Working out of West Tisbury on Martha's Vineyard, McCullough reads his manuscripts to his wife of 22 years, Rosalee, 44, when she is not keeping an eye on their five children, ages 21 to 8. Recently McCullough talked about the canal's history and the ongoing debate with Gail Jennes for PEOPLE.

Why did the United States decide to build the Panama Canal?

A passage through the land mass of the Americas is the oldest dream in American history, dating back to Columbus and the search for a passage to India. But what dramatized the tremendous potential saving in time and distance of a canal was the voyage of the battleship U.S.S. Oregon during the Spanish-American War. It took 67 days to steam from San Francisco round the Horn in savage weather, but it arrived in time for the Battle of Santiago Bay in Cuba. The voyage covered 12,000 miles vs. only 4,000 miles by way of an isthmian canal. As a demonstration of the military importance of a canal, the voyage of the Oregon was made to order.

But the U.S. did not start the canal?

No, it took the Frenchman Ferdinand de Lesseps, and advances in hydraulics including the steam engine, to launch the canal. About a third of the work on the Panama Canal was done by the French, and they lost some 20,000 lives there before the French company went bankrupt in 1888.

Why was the Canal Zone "the hardest-won 50 miles in history"?

Five hundred people died for every mile in building the canal. It involved more men, money and equipment than any prior human undertaking on earth, except war. Every bottle of quinine, locomotive, steam shovel, all the cement and dynamite had to be shipped there. Those locks surpassed anything ever built, including the pyramids. Rainfall is measured in feet, not inches. Disease was so prevalent, it was worth your life to set foot there: malaria, yellow fever, cholera, typhoid fever, even bubonic plague. Including the French effort, the cost was $639 million in turn-of-the-century dollars. For 1977 dollars, you would have to multiply by a factor of about 10.

You have said the "root causes" of the present canal controversy are in the past. What are they?

The engineering of the Panama Canal is a triumph of human intellect and perseverance. But the diplomacy was imperfect. Had Roosevelt been a little more patient, a little less impulsive and not seen the Colombians as a second-class power, we would have had a sound treaty with Colombia, and our canal today would be in the province of Panama in the Republic of Colombia.

Then U.S. support of the Panama revolution in 1903 was a grave disservice to Colombia?

Our implicit support of the Panama revolution did Colombia the greatest disservice in its history. Having lost its most important province and one of the world's great crossroads, Colombia had every right to bitterness toward the U.S. The disservice to Panama was our haste to sign a treaty written and pushed by its special representative, Bunau-Varilla, who was not Panamanian but French, even before Panama's representatives arrived in Washington. The treaty was decidedly in our favor.

What was the U.S. role in the Panamanian revolution?

We shouldn't have any great guilt about it. Without our implicit support, the revolution would never have been tried or succeeded. But there were no shots fired or bloodshed, except the death of one man in a laundry when a Colombian gunboat lobbed a token shot into the city of Panama. The revolution was not a White House plot. We simply let it happen. Many feel we perpetrated a terrible wrong back then which we can right by giving the canal to Panama. Logically we should probably give it to Colombia. Whether the canal goes to Panama is vital, but not from some misty sense of guilt.

Did Teddy Roosevelt "take" Panama?

Many years after he left office, TR said: "I took Panama and let Congress debate me, and while they were doing that, I built the canal." It was partly in jest; it got a big laugh. Actually, McKinley, Roosevelt, Taft and Woodrow Wilson built the canal, which opened in 1914. But Roosevelt put his stamp on everything. He stepped in to assure the success of the revolution. He decided against the sea-level-canal idea in favor of man-made lake and locks; otherwise the canal might have been a failure. He backed Dr. William Gorgas in the extermination of yellow fever and malaria mosquitoes. Finally, as the first President to leave the U.S. while in office, he went to Panama in the worst of the wet season. Teddy splashed around in the mud, tramped through the rains and had a glorious time. From then on there was no problem about the popularity of the canal. It had become a great American crusade. It really is the Roosevelt canal.

What about the Canal Zone today?

This 10-mile-wide strip is a U.S. government reservation in which only employees of the Panama Canal Co. and their families can live. It's a company town, with no representation in the U.S. government or any of its own. Most of those who wish to keep the Zone as it is are conservative, but it is a model socialistic state: stores, transportation, employment, schools—everything is under the control of the U.S. government.

How important is the Panama Canal to the U.S. and the world?

President Carter used the correct word: "vital." People could argue endlessly as to its importance now in contrast to when it was finished. But transits have been increasing steadily and now total about 12,000 ships a year, and by the year 2000 will probably be between 17,000 and 20,000 ships. When the canal was shut down last year during the pilots' strike, the estimated daily loss was $2 million. Some aircraft carriers cannot go through the canal; they are too large for the locks. But our missile-firing destroyers can. About 70 percent of the strategic material in Vietnam went through. And now so does Alaskan oil.

What is your reaction to the two current treaties?

If I were a U.S. senator, I could not tell you at this point how I would vote. I think we must have a new treaty. The original was badly engineered. But we must not replace it with another flawed one.

What are some of your reservations?

The important thing is to keep the canal operating and serving the world. Making Panama a full partner in that operation is essential. But our participation should not cease entirely; our right to defend the canal if necessary must be maintained. I am not sure this new treaty is the one we should sign, but it would dissolve a potentially explosive situation with the Panamanians. I can see a tremendous disadvantage in our relations with Latin America if we don't sign it. But I don't think we will be loved instantly all over Latin America if we do.

What other reservations do you have?

I'm not satisfied that it's necessary to be as generous financially as the treaty calls for. And I'm constantly annoyed by people who talk about giving the canal back to Panama. You can't give back something that never belonged to them in the first place.

There have been warnings that Panama could nationalize the canal, demand that we leave, and thus open the door to Castro or Soviet influence.

There's an element of reality in those concerns. People against change in Panama are generally better informed than people who have given at least lip service to the new treaties.

Will the treaties pass the U.S. Senate?

I think if the vote were today the treaties would not pass. We must keep an open mind in the months ahead, because this is a very complex subject which must be approached with intelligence and fairness. It will be unfortunate if it becomes an ideological shouting match. This current controversy involves American pride in what we built at Panama, which is justified. It involves Panamanian pride in their nationality, sovereignty, their place in the sun, which is also justified.