The year ahead will bring no shortage of international issues that will keep the US and its partners occupied, ranging from the fight against ISIS to the implementation of the Iran nuclear agreement.
But there's been another serious impending crisis building in the background — one involving terrorism, nuclear weapons, interstate power politics, and an important US ally.
The crisis isn't likely to explode in 2016. Its eruption is currently far from inevitable. But there's a good chance, depending on what happens in 2016, that it could become more likely.
As Georgetown University associate professor C. Christine Fair explained in a December 21 article for Quartz, Pakistan's nuclear arsenal is growing at an alarming clip, with Islamabad acquiring some 20 additional nuclear warheads a year.
Worse, the country is developing weapons for "tactical" or battlefield use, believing that it can use low-yield nukes to turn back any potential blitz through the country's territory.
Pakistan, which has an estimated 120 warheads, is now on pace to have the world's third-largest nuclear arsenal by number of warheads within the next five to 10 years. It could place them third only to the US and Russia, according to an August 2015 Carnegie Endowment for International Peace analysis.
Fair's article builds off the Carnegie analysis, in which arms-control scholars Toby Dalton and Michael Krepon argue that Pakistan has oriented its nuclear program around maximizing warhead production and expanding its arsenal's range of potential real-world uses.
As Krepon explained in an article for Arms Control Wonk, Pakistani military thinkers are leaning toward a doctrine of "full-spectrum deterrence." In such a doctrine nuclear weapons are thought of as a way of preventing or neutralizing tactical threats, as well as a way of deterring an Indian nuclear or conventional attack large enough to threaten Pakistan's survival as a state.
But the real measure of the country's dedication to developing its nuclear arsenal can be seen through the opportunity costs of the weapons program: As Krepon writes, Pakistan, which is relatively poor and undeveloped, decided to construct four plutonium production reactors for its nuclear-weapons program, instead of investing in civilian nuclear energy, as India has. According to Krepon, India is only producing about five warheads a year.
It's inherently dangerous for just about any country to grow out its nuclear arsenal, but Pakistan represents a unique case.
Pakistan has a powerful and shadowy security and intelligence apparatus that's had a historically vague and often downright hostile relationship with the country's civilian leadership. There have been longstanding concerns over Pakistani nuclear command and control. In 2011, journalists Jeffrey Goldberg and Mark Ambinder reported that the Pakistani military often transported warheads in unmarked and lightly guarded civilian-style trucks.
India and Pakistan are no closer to resolving their dispute over the status of Kashmir — and if anything, the rivalry between the countries has actually gotten worse.
As Krepon wrote, "We pay attention when firing across the [demilitarized zone] on the Korean peninsula occurs for a day or two — and rightly so. Firing across the Kashmir divide now occurs every week."
There's also a terrorism angle to Pakistan's weapons program. As Fair recounted, one major effect of Pakistan's nuclear arsenal has been to restrain India's options in countering Pakistani-sponsored jihadist attacks inside of India.
In December 2001, three years after Pakistan's first test of a nuclear weapon and two years after its defeat in the Kargil War with India, Pakistani-sponsored terrorists from the jihadist group Lashkar-e-Taiba killed nine people in an attack on the Lok Saba, India's parliament building in Delhi.
Sponsoring an attack on an enemy's parliament is one of the most heinous possible breaches of international order. As Fair recalls, India mobilized its military but did not retaliate, partly because of its fear sparking a potential escalation with an unpredictable nuclear-armed state.
More recently, Lakshar-e-Taiba carried out a November 2008 attack in Mumbai in which 172 people were killed — a state-sponsored assault on an enemy city that still didn't result in continued armed conflict between India and Pakistan.
Pakistan's nuclear arsenal provides the country with a strategic umbrella needed to continue aiding terrorist groups, including ones fighting the Afghan government and its NATO partners.
Command-and-control concerns, along with the possibility of nuclear weapons falling into the wrong hands in a chaotic region, have turned the relationship between the US and Pakistan into something of a protection racket. The US is funding and supporting an opaque and intransigent government simply to maintain the currently unstable nuclear-security baseline.
The growing Pakistani stockpile also feeds into a hazardous regional dynamic. As Fair explains, Pakistani battlefield nuclear weapons are meant to counter a new Indian strategic doctrine that calls for nimble offensive operations on a compressed timescale in the event of a future provocation.
Pakistan is in possession of a nuclear arsenal and the means to create warheads at a rapid clip — at the same time the country's military leaders are seriously thinking about the weapons' applicability in combat.
Pakistan has already proven to be a proliferation danger, as nuclear expertise from the country was crucial in advancing the programs of Iran and North Korea. The country's nuclear weapons raise the stakes of Pakistan's ongoing and perhaps worsening confrontation with India, while the growth of its arsenal might make it likelier that the weapons will be used — and likelier that they'll fall into the wrong hands.
In the coming year, policymakers the world over should think about how this problem can be reigned in. Kerpen and Dalton argue for international recognition of Pakistan as "a normal state that possesses nuclear weapons," as long as it "pulls back from full-spectrum defense," separates its civilian and military nuclear facilities, submits to certain stockpile controls, and signs the Comprehensive Nuclear Test-Ban Treaty.
As Fair noted, some in the Obama administration have floated the idea of a nuclear deal with Pakistan that would slow the growth of the Pakistani arsenal in exchange for some degree of official US recognition of the country's nuclear rights. (The US finalized a similar agreement with India in 2008.)
Getting Pakistan to agree to any nuclear concessions might prove difficult, particularly in light of the concessions that Iran won from the international community in the July 2015 nuclear deal — gains that Tehran achieved without possessing even a single warhead. Given this precedent, a nuclear breakthrough with Pakistan might have to come as the consequence of some unforeseen event — like another Kashmir escalation, or a political transformation inside of Pakistan.
But the Pakistani nuclear arsenal should still be high on the world's agenda in 2016 — even if it remains a challenge that doesn't get a lot of public attention