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What conditions could make an army have its MBT be amphibious with all the tradeoffs that has? While many nations have made amphibious tanks, none (other than sweden that one time) have done it for their MBT. This can't be through handwaving trade-offs like having some super materiel so that a regular tank can be amphibious. It doesn't have to be always amphibious, it can use something like a flotation screen to be amphibious. The technology level is near future. Combat with rivals using modern or semi-modern equipment is the main scenario militaries plan for. Not all militaries have to do this, just some. Although small parties of raiders with 30-150 men & light tank support are used by some nations alongside conventional mechanized units.

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    $\begingroup$ A Main Battle Tank for a country is entirely based on that countries requirements. An open desert warfare tank is different to a cold forested area. If your MBT is amphibious the only conclusion is that there are so many water obstacles that "regular" MBT's aren't practical. If your Abrahams or T-90 can't cross the bridges, has to be driven hundreds of kilometers around to get to their objective and can't even reach 80% of the country then its not a useful thing to bring. $\endgroup$
    – Demigan
    Aug 24, 2021 at 8:10
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    $\begingroup$ It should be patently obvious why an army has nothing but amphibious tanks: they need the feature. $\endgroup$
    – RonJohn
    Aug 24, 2021 at 16:17
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    $\begingroup$ A dry season and a very wet season, so that you either need to have fully amphibious vehicles or swap out your entire fleet twice a year? (Not posting a full answer because I'm not sure if this is far enough in the future that you can mess with the climate that much.) $\endgroup$ Aug 24, 2021 at 16:22

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Terrain:

If you are adapting your tanks to amphibious, there is only one logical reason: the terrain calls for it. It would only need to be the terrain for the country that made the decision itself - imagine a Pacific-based superpower centered on trade and controlling numerous islands.

  • Lack of functional roads: If your world has no roads, then moving tanks on them is worthless. But for much of history, rivers WERE the roads. a broad network of interconnected rivers and lakes without roads means the tanks can be easily maneuvered by water while still being able to fight on land.
  • Terrible terrain: There's a reason the French thought the Ardennes forest was impassible. It was a mess of trees with few routes through. Only careful planning overcame this obstacle. But it wouldn't need to be trees. Rocky terrain would be just as problematic. And large swaths of fine ash might not be much better. An amphibious design lets you go through water when it's there, and the tracks for an amphibious vehicle might be amenable to fine ash and sand similar to what the US faced on Iwo Jima.
  • Limited land/many islands: The efficacy of tanks is seriously limited in a war involving many coastlines, numerous islands, and little open land. The only workable armored vehicles would be those you can get to the fighting. While there were Sherman tanks in the Pacific, few people remember them as it was challenging to deploy them.
  • Landmines: In a world where the use of landmines has proliferated (including air-dropped area denial weapons), mining rivers and lakes may have lagged behind. While every road and open field is a giant booby trap, your clever engineers anticipated this problem and have moved their vehicles to the water.
  • Improved tactical flexibility: Coming from the land of 10,000 lakes (and it's actually more than that) I can see the advantage of taking a short-cut. The ability of your armored column to go across the lake or river instead of around it has ALWAYS been the advantage of amphibious vehicles. And if your support vehicles for the MBT are also amphibious, the whole combined arms philosophy comes into play.

And there are some factors not as directly centered on terrain:

  • Crippling global fuel shortages: Heavy vehicles burn fuel. Amphibious vehicles are lighter (by necessity). An Abrams battle tank is a cute decoration on a street if you can't afford to keep it in fuel. I can even imagine a situation where lighter, low-profile tanks moving by sea deploy sails for travel between islands on patrols, and engage engines for maneuvering/fighting. Lighter vehicles might be able to employ power systems that are more ecologically friendly than most big tanks. Alternative fuels in smaller engines with less requirements for horsepower might be the only ones available. In the same way the Harrier might be the only jet flying after the opening rounds of a major war, your lighter tank burning ethanol or methane might be the only MBT that is still practical to run after all the world's oil fields are blown up.
  • Armor VS. Weapons: The old balancing act between armor and weapons has been shifting towards weapons slowly for a long time. As light missiles come to dominate the battlefield, the need for an enormous MBT becomes harder to justify. But just as battleships kept being made long after they were irrelevant, your rich world governments are entranced with the idea of huge tanks that are invulnerable. Not only are your amphibious tanks able to go on water, they're generally more maneuverable and faster. When the game is "get him first," speed and maneuverability are key.
  • Traditions: A nautical society might favor ships over tanks, so the tanks end up more like small ships. A military that has already spent a lot of money developing amphibious tech for other vehicles is more likely to apply the same engineering to their MBT so the designs are similar. Similarly, if you have amphibious trucks, APC's, fuel vehicles, jeeps, and light armor, why not have a MBT be amphibious as well so they work together better?
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    $\begingroup$ I would add: you need the ability to deploy from littoral watercraft, in conditions where they cannot approach the shore very closely. Perhaps there are offshore reefs that prevent close approach; so the tanks need to drive off the ship into the water, maybe drive over the reefs, and then swim the rest of the way to shore. Current landing-craft deployment is pretty advanced so you might need to contrive some very specific conditions to make this necessary. $\endgroup$
    – CCTO
    Aug 24, 2021 at 14:56
  • $\begingroup$ Do amphibian tanks use more fuel when swimming or when moving on land? $\endgroup$ Aug 24, 2021 at 22:33
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    $\begingroup$ Pretty sure they generally use more fuel in the water, unfortunately, but I have this cool vision in my head of three guys and a sail on top of an amphibious tank. They should use less fuel on land than a MBT due to weight, but sometimes these things are tricky (separate drive systems adding weight, etc.) It would depend on the design. Integrated will be heavier, a couple floats and a pair of outboard motors not so much. A lot of add-ons tend to get tacked on to the designs by governments who want fancy functions. $\endgroup$
    – DWKraus
    Aug 24, 2021 at 23:16
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Because your nation is like Indonesia

Indonesia's territory is a collection of large islands and small islets sitting near some of the world's largest sea trade routes. Piracy(the old-fashioned sort) is an ongoing concern, as are religious extremist terror groups. Even in peacetime, the aforementioned groups can set up camp on any islet and they would have to be removed via an amphibious operation.

However, Indonesia did it the other way round. Instead of amphibious MBTs, their concept is a tank boat that fulfills the role of both landing craft and assault gun. Behold the Antasena-class combat boat, or Combat Boat X-18. I know this isn't quite the same as amphibious MBTs, however the concept can easily evolve in that direction once hostile nations get involved. The need to fight other tank boats and conventional armour would plausibly result in developing into a more tank-like vehicle.

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  • $\begingroup$ Or something like the mid-eastern parts of Finland, where there's a bucketful of lakes. Might need a bit more water (and less bridges), but with enough land around it makes more sense to start with tank and not a boat. $\endgroup$
    – ilkkachu
    Aug 24, 2021 at 20:15
  • $\begingroup$ Yes, but no. Tiny islands controlled by China is why After Nearly a Century, the U.S. Marine Corps Is Ditching Its Tankspopularmechanics.com and instead are investing in rockets and missiles. Tanks are for land wars in Asia after establishing air superiority. $\endgroup$
    – Mazura
    Aug 25, 2021 at 0:35
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Have a look at what causes the development of Main Battle Tanks initially

A Main Battle Tank is a little bit of a generic term, with the actual definition as:

a self-propelled armoured fighting vehicle, capable of heavy firepower, primarily of a high muzzle velocity direct fire main gun necessary to engage armoured and other targets, with high cross-country mobility, with a high level of self-protection, and which is not designed and equipped primarily to transport combat troops

However in terms of military procurement they are the confluence between several different factors:

  • Having an optimised combat platform that provides the most amount of speed, weight, firepower and armour in comparison to a similarly optimised enemy platform
  • Having a configuration that takes advantage of standardised manufacturing to minimise manufacturing costs and reduce training costs for specialised purposes (ie. by developing advanced armour, this development can assist in multiple product offerings)
  • Having ability for weapons manufacturers to 'sell' to multiple countries instead of each country having it's own unprofitable companies. Ie. by standardising tank specifications, a technologically advanced tank can be made for many situations that other countries may encounter. Many optimised MBT tanks are sold to countries that cannot afford the colossal R&D costs required.

So in order for your MBT's to be amphibious:

  • There needs to be an enemy that also needs to use amphibious tanks
  • There needs to be a desire by multiple countries to require amphibious tanks such that weapons manufacturers integrate it into their offering
  • There needs to be an almost universal need in terms of tactics and training to reduce the land-only tanks to limited markets

So perhaps, as speculation:

  • A common or steady conflict between countries that have limited land, such as island or swamp nations, forming a major market
  • A major calamity such as sea level rise reducing amount of land for land-only tanks to a degree that no military wants them, and companies have no need to optimise manufacture and develop for them
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For starters, you should be aware that almost all (if not all) MBTs have adaptations for deep wading - instead of swimming on the water, they can cross water by driving on the bottom. For this those tanks have attach points for massive pipes to provide air for the engine and crew to a depth of 3-4 metres.

Then, I assume you want an actual MBT, a tank capable of fighting "ordinary" land only MBTs, rather than a "glass cannon" type light tank with minimal armour but offensive weapons of MBT (as those actually are often amphibious).

You will end with a much bigger vehicle, as you need to provide flotation capability for all that armour (50-60 tons) as well as alternative propulsion. If you do not armour your flotation devices, the tank will stop being amphibious as soon as it enter combat. If you do, the tank could cost 3 times as much as ordinary tank.

Besides, amphibious tank will not be able to cross large bodies of water (like from island to island) as it just barely stick out of surface so can't operate on anything other than very calm water.

You have to question yourself, why do you need amphibious tanks:

  1. Broken and muddy country with a lot of lakes - You will probably be better of with cheap and mobile amphibious light tanks as in such terrain you can easily fire from protected positions.
  2. multiple small rivers - deep wading give the same for much lower cost
  3. large rivers - landing barges may be deployed for a lower cost and, if it is your land, river ships with tank turrets may provide fire support.
  4. island country - gunships/avisos (today called tank boats) provide ability to sail in most sea conditions and engage targets on land. Plus actual warships may engage shore targets as well. If you need to drive deeper inland, ordinary MBT can be delivered by landing craft or hovercraft etc.
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At this point in the war, the amphibious tank is all they can make.

https://en.wikipedia.org/wiki/Malyshev_Factory#Tank_production

Shortly before the German invasion of the Soviet Union the KhPZ started series production of the T-34, the most-produced tank of World War II. Series production began in June 1940 in Kharkiv, and later in the Stalingrad Tractor Plant and Krasnoye Sormovo Shipbuilding Plant. In 1941, due to German advances, the factory and design shops were evacuated to the Ural mountains;[1]

There are no reasoned calculations here. @DWKraus has covered those. This is a matter of necessity and desperation. The MBT for this nation is an amphibious tank because that is the factory these folks still hold. Factories that made other tanks have been captured or destroyed. Resources are reallocated to the amphibious tank factory that they still hold.

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