At the height of the Cold War, the United States and the then-Soviet Union possessed around 70,000 nuclear warheads between them — enough to destroy or poison every square centimeter of inhabited land on Earth several times over.

Read more: Nuclear buttons: How easy is the beginning of the end?

Somewhat ironically, the concept of "mutually assured destruction" (MAD), whereby the full-scale use of nuclear weapons by two or more opposing sides would cause the complete annihilation of both the attacker and the defender, was and is a guarantor of peace.

Now, US President Donald Trump has unveiled his plan to modernize the country's nuclear capabilities. The Nuclear Posture Review (NPR) makes the case for the "requirement that the United States have modern, flexible, and resilient nuclear capabilities that are safe and secure until such a time as nuclear weapons can prudently be eliminated from the world."

At the heart of the new doctrine is the desire to expand America's nuclear options, but not the overall size of its arsenal. The development of low-yield nuclear bombs, also known as tactical "nukes," would allow the US to respond more flexibly to future challenges and comes largely in response to Russia's recent actions, notably its annexation of Crimea in 2014.

So what does the revised strategy entail and how much will it cost?

US nuclear policy and strategy is based on four pillars:

— Deterrence of nuclear and non-nuclear attack

— Assurance of allies and partners

— Achievement of US objectives if deterrence fails

— Capacity to hedge against an uncertain future

To be able to meet those objectives, the US plans to upgrade its so-called strategic nuclear triad, which consists of:

Submarines (SSBNs) armed with submarine-launched ballistic missiles (SLBM)

Currently, the US operates 14 OHIO-class SSBNs based in Bangor, Washington, and Kings Bay, Georgia. These submarines can deploy 288 Trident II D5 SLBMs. The Navy plans to replace the current fleet with the COLUMBIA-class submarines starting in 2031. According to the Government Accountability Office (GAO), acquisition costs could exceed $128 billion (€103 billion).

Land-based intercontinental ballistic missiles (ICBM)

The ICBM force consists of 400 single-warhead Minuteman III missiles that are deployed in underground silos located at F.E. Warren Air Force Base, Wyoming, Malmstrom Air Force Base, Montana, and Minot Air Force Base in North Dakota.

The US government has given the green light for the Ground-Based Strategic Deterrent (GBSD) program, which would replace Minuteman III missiles in 2029. The 450 ICBM launch facilities will also be upgraded as part of that program. Conservative estimates put the acquisition costs at $85 billion and the total life-cycle cost at $238 billion.

Strategic bombers with gravity bombs (free-fall bombs that don't use a guidance system) and air-launched cruise missiles (ALCMs)

The bomber element of the triad consists of 46 nuclear-capable B-52H bombers based at Minot Air Force Base, North Dakota, and Barksdale Air Force Base, Louisiana, while there are 20 nuclear-capable B-2A "stealth" bombers based at Whiteman Air Force Base in Louisiana. With an eye on phasing out parts of the conventional and nuclear-capable bomber force beginning in the mid-2020s, the US has launched a program to develop the next-generation bomber, the B-21 Raider. At the same time a new nuclear-capable cruise missile — the Long-Range Standoff Weapon (LRSO) — will replace the existing ALCMs. The Congressional Budget Office (CBO) estimates that the B-21 program will cost $97 billion.

According to a CBO report published in October last year, the nuclear weapons spending plans could cost taxpayers $1.2 trillion in inflation-adjusted dollars between the fiscal years 2017 and 2046.