In February, the Pentagon released a new Nuclear Posture Review (NPR), a comprehensive overview of the nation’s nuclear forces, their disposition, and plans for the future. Here, Belfer Center experts share their assessments of this NPR and the future of America’s nuclear capabilities.

In a "policy memo" to President-elect Donald Trump, Graham Allison and Dimitri K. Simes write: "The two Chinese characters that make up the word “crisis” can be interpreted as meaning both “danger” and “opportunity.” Russia today offers your administration not only a serious challenge but a significant opportunity.

Russia is no longer the Evil Empire the United States confronted over decades of Cold War. Nonetheless, Russia remains a player whose choices affect vital U.S. interests profoundly across the agenda of global issues. First and foremost, Russia remains the only nation that can erase the United States from the map in thirty minutes.

The attacks in Brussels last month were a stark reminder of the terrorists’ resolve, and of our continued vulnerabilities, including in an area of paramount concern: nuclear security.

The attackers struck an airport and the subway, but some Belgian investigators believe they seemed to have fallen back on those targets because they felt the authorities closing in on them, and that their original plan may have been to strike a nuclear plant. A few months ago, during a raid in the apartment of a suspect linked to the November attacks in Paris, investigators found surveillance footage of a senior Belgian nuclear official. Belgian police are said to have connected two of the Brussels terrorists to that footage.

The defining question about global order for this generation is whether China and the United States can escape Thucydides’s Trap. The Greek historian’s metaphor reminds us of the attendant dangers when a rising power rivals a ruling power—as Athens challenged Sparta in ancient Greece, or as Germany did Britain a century ago. Most such contests have ended badly, often for both nations, a team of mine at the Harvard Belfer Center for Science and International Affairs has concluded after analyzing the historical record. In 12 of 16 cases over the past 500 years, the result was war. When the parties avoided war, it required huge, painful adjustments in attitudes and actions on the part not just of the challenger but also the challenged.

The latest sticking point in nuclear negotiations with Iran has little to do with the issues that have captivated attention in Washington—centrifuges, uranium stockpile and inspections of military sites. Instead, it has focused on the intricacies of sanctions: the Iranian delegation has demanded that a United Nations embargo on conventional weapons and ballistic missiles be lifted. While the United States rejects such a concession, the Russians have reportedly broken ranks and support Iran’s position.

Diplomats are extending Iran nuclear negotiations into overtime this week, and American lawmakers are preparing for mandatory congressional review. As they decide whether to vote yes or no on a possible deal, they should remember the sage advice of the German philosopher Friedrich Nietzsche, who observed that the “most common form of human stupidity is forgetting what one is trying to do.” I have a framed version of that quotation in my office and try to think about it every day.

In the case of Iran’s nuclear program, what is the U.S. trying to do? In a sentence: “Stop Iran verifiably and interruptibly short of a nuclear bomb.” No agreement, no airstrike, and no other option anyone has identified can give 100 percent assurance that Iran will not acquire a bomb. The U.S. does not have 100 percent confidence today that Iran has not already built a bomb, or bought a weapon from North Korea (from whom it has certainly purchased missiles). The question members of Congress must answer is whether the deal the U.S. and its P5+1 partners have negotiated is more likely to prevent Iran’s acquiring a bomb for the lifetime of the agreement than any feasible alternative.

As the policy community prepares to assess an agreement between the U.S. and its P5+1 partners and Iran, Senate Foreign Relations Committee Chairman Bob Corker asked me to review the history of analogous agreements for lessons that illuminate the current challenge. In response to his assignment, I reviewed the seven decades of the nuclear era, during which the U.S. negotiated arms-control treaties, including the Non-Proliferation Treaty of 1968; strategic arms limitation talks and agreements from SALT to New Start; the North Korean accord of 1994; the agreements that helped eliminate nuclear weapons in Ukraine, Kazakhstan, and Belarus in the early 1990s; and the pact that eliminated the Libyan nuclear weapons program in 2003.

Among many lessons and clues from this instructive history, five stand out