I'm watching The Pacific from HBO now, and it's striking me how similar island hopping battles in WW2 Pacific theater were to the Vietnam War.

I couldn't disagree with this assessment more. To understand why I say that, we must look at the context of these battles. And understand that this topic is pretty complex, so this is a cursory explanation, as a full answer necessitates looking at everything from tactics on the ground to strategic necessity in the larger theater.

WW2 in the Pacific Theater involved major strategic battles, including naval ones like the Battle of Midway, where the importance of the battle wasn't in terms of the # of sailors or soldiers lost, but in the strategic importance of destroying four Japanese aircraft carriers - a precious commodity in a war on the seas.

The island hopping campaign was designed to take strategically important islands for a variety of reasons: airfield suitability, guarding vital supply routes to other Japanese strongholds, and to create staging grounds for future operations. Sure, some islands were more important than others, but that was the general process besides selecting and assault islands. Certain Japanese strongholds - such as the one on Rabaul, a major base of Japanese operations - were never directly attacked, but instead avoided and encircled and bombarded, making it effectively useless.

In sum, there was a slow and steady push towards the Japanese main islands, with the ultimate objective of defeating the Japanese Empire in mind.

Vietnam, however, could be considered a 'police action' and has often been contrasted directly to WW2 precisely because it was so different.

In Vietnam, the goal wasn't to defeat North Vietnam - it was to preserve the South Vietnamese government. That meant battling communist insurgents in the country side, the Vietcong, and in later years, actual North Vietnamese Army (NVA) forces that crossed the border.

Attacks on North Vietnam primarily came from the air, with strict rules of engagement (ROE) to prevent a larger escalation of the war, as North Vietnam was supported by the Soviet Union and China (until later in the war).

Battles in Vietnam came in many flavors - some were indeed assaults on Vietcong strongholds or supply areas, but others were on patrol and coming into contact with enemy forces. Still others, like the Tet Offensive, came when the Vietcong themselves attacked.

The show mentions that Iwo Jima was the only battle where Japanese casualties were lower in number than Allied casualties, which made me wonder why the Vietnam war was a complete opposite.

For one, the American forces in Vietnam had established air superiority, and never had a credible threat of Vietnamese air attacks on ground troops. In addition, the Vietcong didn't have the sustained fire support the American forces had: airpower and artillery power.

You should note too that in the rest of the Pacific, Japanese casualties exceeded American casualties. Iwo Jima was a special case for a variety of reasons, including the fact that it was heavily fortified island with virtually every inch of the island pre-sighted by artillery on the island. The volcanic ash made it hard to nearly impossible to dig in. The commander of Japanese forces, General Kuribayashi, also dissuaded soldiers from launching suicide banzai attacks on American forces, preferring to drag them into the teeth of prepared defenses and fight inch by inch for the entire island.

The jungle, the suicide tactics, the climate, the guerrilla forces seem extremely similar comparing the Japanese soldiers with the Vietnamese.

Vietnam - while having hostile terrain too (a jungle environment) - saw little in the way of fixed fortifications by the Vietcong, who preferred, as you stated, guerilla tactics.

The Vietcong also didn't have the same organization of a full military, which the Japanese did, and didn't have the same heavy equipment the Japanese had on many of its islands (like on Iwo Jima and Okinawa), namely in the form of concentrated artillery batteries and what not.

The US was also primarily on the offensive in the island hopping campaign. Again, engagements in Vietnam varied in how they started, but both sides tended to have surprise to an extent that wasn't existent in the Pacific: the use of helicopter forces by the US allowed unprecedented amounts of mobility. Meanwhile, bombardment of islands could last days or even weeks - and preparations for digging in by the Japanese would precede that by months or even years.

And before I even forget - amphibious assaults! They are complex and dangerous and history is riddled with failed ones like Gallipoli and Dieppe. That the US Navy and Marine Corps turned it into an art in the Pacific shouldn't cloud how dangerous and costly these operations were.

In sum: the Japanese didn't share all that many tactics with the Vietnamese, the battles were often on very different landscapes, the US had a consistent superiority in tech and firepower throughout the Vietnam War, the Japanese had far more time to prepare defenses on islands as the war dragged on while the Vietcong and later NVA were primarily on the offensive, and so on.

Finally, the political consideration:

Why was one a difficult, but lopsided, US victory while the other was a difficult, but much more deadly and ultimately losing war for the US?

Ultimately, political considerations come into the way. WW2 was a no-holds-barred total war: the full industrial might of the US was brought to bear, and by war's end, the US had over a hundred aircraft carriers of all sorts of flavors, and every option was considered for defeating Japan.

In such a war, things like strategic resources, strategic weapons, etc. all come into play. The Imperial Japanese Navy was withered down and beaten into submission over the years so that by 1945, it could no longer mount a credible threat to American power. Japan, being an island nation, relied upon its sea routes to feed and supply itself - and this was being challenged by American submarines.

That's not to say that American victory came easy - for instance, on Okinawa, over 12,500 Americans were killed in action in just 3 months of fighting (April 1st to June 22nd).

Vietnam, in contrast, pales in comparison in size and scale: 1968 was the only year that exceeded all those killed on Okinawa, with 16,899 killed in 1968. That's right - in any other year of the Vietnam War, more people were killed on Okinawa in 83 days than in the entire year of 1967 or 1969.

So why the defeat for the US? It largely came down to the politics of the war: with no concrete objectives in mind, no strategic/steady progress in the war that could be easily determined, and waning public support at home, the US ultimately withdrew from South Vietnam. Whereas one could follow along on a map in WW2 and see American forces get closer and closer to Tokyo, Vietnam was an argument of how much longer could the US sustain defending South Vietnam from Vietcong and North Vietnamese forces. The answer was: not indefinitely.

There are a ton of sources involved, so here's a sampling of some that cover this very broad topic:

• Toland, John - The Rising Sun

• Allen, Robert E. - The First Battalion of the 28th Marines on Iwo Jima: A Day-by-Day History from Personal Accounts and Official Reports

• McNamara, Robert S., with James Blight, Robert Brigham, Thomas Biersteker, Herbert Schandler - Argument Without End: In Search of Answers to the Vietnam Tragedy.

• Villard, Erik - The 1968 Tet Offensive Battles of Quang Tri City and Hue