D-Day was the largest amphibious invasion in history — and likely the last. Here's why.

• D-Day on June 6, 1944 was a gargantuan feat. Is it the last invasion of its kind?
• In World War II, the best way to stop an amphibious invasion was before troops reached the beach.
• Modern weapons and surveillance systems give a defender more power to accomplish that.

D-Day is more than the largest amphibious invasion in history. Even 80 years after the battle, it still resonates as an epic of courage, endurance, and prodigious effort.

But was D-Day the last invasion of its kind? Could such immense resources be mustered again in a modern-day version of "Saving Private Ryan"-style landings?

World War II marked the zenith of wars waged between mass armies, an era that began in 1792, and modern weaponry, from guided missiles and spy satellites to nuclear bombs, has obviated large-scale amphibious invasions.

Even by the standards of WWII, the Allied invasion of Normandy on June 6, 1944, was gargantuan. Operation Overlord embraced more than 2 million personnel, 13,000 aircraft and 7,000 vessels from a dozen nations. Six infantry divisions — three American, two British and one Canadian — would hit the beach simultaneously, while three airborne divisions — two American and one British — landed in the German rear.

Some 160,000 troops splashed ashore that Tuesday morning, including around 75,000 American and 61,000 British soldiers, plus 13,000 Allied paratroopers descending from the skies. And these were only the tip of a spear across Europe and the Pacific: the US Army expanded to 11 million personnel during the war — including 2 million in the European theater alone — while the British Army grew to 3 million strong.

These forces today are only a fraction of what they were then. The entire US Army only numbers around 450,000 active-duty soldiers; for Britain, it's 76,000 and shrinking. There were 1,200 warships escorting the D-Day convoys of soldiers and equipment: today's US Navy has just 290 major warships and amphibious vessels spread around the globe, while the Royal Navy is down to 66 commissioned vessels. The Royal Canadian Air Force's 86 squadrons made it the world's fourth-largest air force in 1945, including nearly 40 squadrons that fought at Normandy: today it has just four fighter squadrons with 77 aging F/A-18 Hornet fighters.

D-Day's Allied planners were haunted by two fears: getting ashore despite German coastal defenses – and then surviving the inevitable German armored counterattack from Panzer divisions waiting behind the beaches. This meant there had to be enough transport capacity to land a large assault force and then quickly reinforce it with troops, supplies, and especially the tanks and artillery needed to even the odds in an armored fight.

More than 3,000 landing and transport vessels were tasked with this mission. The US Navy now aims for enough amphibious capacity to land just two Marine brigades on a hostile shore. More than 1,000 American C-47s transport planes dropped three airborne divisions on D-Day. With just over 200 C-17 cargo planes to meet worldwide airlift requirements, the current US Air Force would struggle to airdrop the entire 82nd Airborne Division.

All of which points to a fundamental difference between 1944 and 2024. The major combatants 80 years ago were committed to total war, in which they mobilized their human and industrial resources to the utmost. Today's Western militaries are much smaller volunteer forces, sustained by a defense-industrial base with only a fraction of its 1940s capacity. As shown by persistent shortages of weapons and ammunition in the Russo-Ukraine War, even if enough soldiers could be drafted to mount a Normandy-sized invasion, there wouldn't be enough equipment for them.

In World War II, the best way to stop an amphibious invasion was before the first troops stepped onto the beach. If the defender's navy and air force could destroy or turn back an invasion fleet, the landing would never take place. D-Day could only happen because, after a long and bloody struggle, the German surface fleet had been decimated, the U-boats suppressed, and the Luftwaffe mauled. Its forces defending French coasts were also spread thin because of the necessity of countering an advancing Soviet force of 140 divisions on the Eastern Front.

Though it must have seemed otherwise to troops splashing ashore under heavy fire, the very fact that the invasion was happening meant the battle for the beach already tipped in favor of the Allies. They could count on the naval bombardment and bombing runs to target German strongpoints and hammer its forces massing for a counterattack.

Isolated from reinforcement and resupply by US ships and aircraft, Japanese troops on Pacific islands, by contrast, could fight to the last man and inflict heavy casualties, but their destruction or isolation was only a matter of time. German counterattacks almost drove Allied landings at Sicily, Salerno and Anzio into the sea, but aided by devastating naval gunfire, the bridgeheads held on.

Hitler placed great hopes in the Atlantic Wall, a fortified belt across 1,700 miles of coastline. The bunkers and machine gun nests did inflict some losses at Normandy, especially during the bloodbath suffered by American soldiers landing on Omaha Beach. But the Atlantic Wall was spread too thin to repel overwhelming Allied force at the landing sites.

In modern strategic parlance, stopping amphibious invasions is part of "anti-access/area denial," or A2/AD. Today's invasion planners worry that coastlines — and waters extending hundreds of miles from the beaches — are becoming no-go zones for warships and transports. Spy satellites and reconnaissance drones can discover an invasion fleet, exposing it to long-range attacks and giving the defender more time to mass troops and firepower in the likely landing zone. Coastal defense weapons include hypersonic anti-ship missiles streaking in at 10 times the speed of sound, GPS-guided cruise missiles and glide bombs, small but stealthy submarines, long-range guided artillery shells, and a variety of aerial and maritime drones. An enemy that has these can threaten the invasion armada and the landing force it launches as it chugs to the beach.

And it's not just major powers like Russia and China that have these arms: even smaller powers like Iran and North Korea could turn beaches into death traps.

There are already signs of this. In the 1982 Falklands War, the Argentine Air Force — armed with bombs and a few Exocet anti-ship missiles — sank six British warships and transports, and nearly derailed the invasion. And in the ongoing Ukraine war, despite initial fears that the Russian Navy would shell cities and land amphibious troops, the Black Sea Fleet has lost two dozen warships and amphibious vessels to anti-ship missiles and small, robotic boats packed with explosives. Russia may have a much larger navy than Ukraine, but it doesn't dare venture closer to the Ukrainian coast.

Of course, it can be argued that technology works both ways. Smart bombs can destroy coastal defenses. Helicopters can ferry troops and supplies from an amphibious fleet hundreds of miles away. Drones likes unmanned tanks and mine-clearing robots can clear beach obstacles.

Yet ultimately, what has really killed massive amphibious invasions is the poisonous mushroom cloud. Even as far back as 1945, after the US atomic blasts at Hiroshima and Nagasaki, nations realized that a single bomb capable of destroying a city could also wipe out an amphibious fleet. The U.S. Navy tests at Bikini Atoll in 1946 vividly demonstrated how a nuclear blast could smash even battleships and aircraft carriers like bathtub toys. Russia for example is armed to the teeth with tactical nukes that could devastate a massed fleet.

This doesn't mean that amphibious operations are obsolete. They are still needed on a planet that is 71 percent water, and where the ocean is often the only feasible way to transport armies. But we should honor the memory of D-Day, because we shall never see another day like it again.

Michael Peck is a defense writer whose work has appeared in Forbes, Defense News, Foreign Policy magazine, and other publications. He holds an MA in political science from Rutgers Univ. Follow him on Twitter and LinkedIn.