NEW LONDON, Conn. — They had retained their IRA title last weekend, dunking unbeaten Washington by a length. But a century and a half of history dictates that Yale’s crew season is judged by the finale. “Last week was the national championship and yet this was mandatory,’’ coach Steve Gladstone was saying Saturday as dusk fell over the Thames River. “It had to be done. Beating Harvard — it had to be done.’’

And so the imposing Bulldog heavyweight varsity squelched its archrivals for the third straight time in their four-mile showdown, taking control in the second mile and cruising to a 2½-length triumph in the 153rd running of the country’s oldest intercollegiate athletic event, winning the upstream race by nearly eight seconds in 18 minutes, 51 seconds.

“We just did what we do everyday,’’ said stroke Sholto Carnegie. “Side by side boats, and you just battle until one cracks.’’

It was the first time since 1984 that Yale had won three consecutive meetings (the 2016 event was annulled after Harvard swamped early with Yale ahead) and it affirmed the Bulldogs’ primacy in a series that the Crimson had dominated for most of half a century.

“They’re able to assert themselves,’’ said Crimson coach Charley Butt. “They’re just very strong and they row very well. They just happen to be stronger than we are right now.’’

Yale had beaten runner-up Harvard by more than three seconds at last month’s Eastern Sprints and finished nine seconds ahead of them at the IRAs, where the Crimson placed fourth behind California. But none of that would have mattered if the Bulldogs hadn’t been able to prevail here.

“These guys win all their races. They win the Eastern Sprints. They win the national championship. I mean, triple jeopardy?,’’ mused Gladstone. “And look what they did. They handled it.’’

Not that their rivals made it easy. The Crimson’s third varsity came from behind to beat Yale’s Sprint champions by nearly three seconds in their two-mile duel. And its JV, which was nearly eight seconds astern of the Bulldogs at the Sprints, fought hard in the final half mile of their three-mile chase to narrow the margin to a couple of lengths. “There is never any quit in any of our boats,’’ Butt observed.

The Crimson varsity, which kept chewing into Yale’s lead last year until the Bulldogs finally closed things out in the last mile, again was determined to hang in for as long as it could. “We wanted to be stubborn and persistent,’’ said coxswain Cole Durbin. “We knew Yale was a fast crew. We wanted to be scrappy. I think we did a good job of that today. All you can ask for is a full effort and I think we gave it.’’

That effort needed to be all-out from the moment the flag dropped beneath the railroad bridge. Harvard knew from experience that it couldn’t afford to be left at the stake boat.

“We’ve seen Yale throughout the season really dominate off the start, especially in that first 20 strokes,’’ said captain Conor Harrity. “That was our goal — to hang with them in those first two minutes and make them suffer.’’

The Crimson actually went up by a seat in the early going but by the half-mile marker the Bulldogs led by half a length and were moving. After 1½ miles Yale had open water and was extending but Harvard dug in and dug deep and began closing in the final mile.

“The beauty of sports is that until it ends you always have a shot,’’ said Harrity, whose seatmates are formidable finishers. “There are a number of circumstances in that last half-mile that could have gone our way but as the race narrows down the chance of that happening is a little bit less.’’

So it was Yale’s day as it was expected to be, if nothing else because losing to Harvard was unthinkable to the blue-shirted oarsmen. “They thought it would be a disaster,’’ said Gladstone. “It would be a bitter way to end the season. This was an obligatory act and they did it. They crushed it.’’