1949 The Soviet Union conducts a successful atomic test in Kazakhstan, and the nuclear arms race between the Soviet Union and the U.S. begins.

1953 President Dwight D. Eisenhower delivers his “Atoms for Peace” speech, suggesting the United States provide nuclear material to other countries for research and civilian use.

1954 The U.S. passes the Atomic Energy Act, to encourage the “peaceful uses of atomic energy,” beginning the provision of nuclear material to countries throughout the world. Until the 1970s, the United States imposed no rules on how those nuclear materials should be secured.

1960 A team from the Joint Committee on Atomic Energy finds little security for U.S. nuclear weapons in Europe, and few safeguards to prevent unauthorized use. Partly in response to their report, President John F. Kennedy later orders locks known as Permissive Action Links (PALs) to prevent anyone from setting the weapons off without an authorized code.

1962 Cuban Missile Crisis: Moscow covertly dispatches approximately 130 nuclear warheads ninety miles from the U.S. coastline.

1964 China tests its first nuclear bomb.

1965 A large amount of highly enriched uranium (HEU) is unaccounted for at the Nuclear Materials and Equipment Corporation (NUMEC) in Apollo, Pennsylvania. The incident provokes a major revision of U.S. rules for accounting for nuclear material.

1967 An official U.S. government study warns for the first time about the realistic threat that terrorists could steal nuclear material and use it to build a nuclear bomb.

1968 The Joint Chiefs of Staff become concerned about China clandestinely introducing nuclear weapons into the U.S.

The modern era of international terrorism begins in the wake of the 1967 Arab-Israeli war, hijackings, and other acts of terror.

1970 The Atomic Energy Commission issues the first U.S. rules requiring security measures for privately owned nuclear material.

1971 “Black September” takes place, as terrorists hijack multiple aircraft almost precisely 30 years before the 9/11 attacks.

1972 The Munich Olympics massacre demonstrates that well-trained, well-armed terrorist teams can strike in major developed countries. United States begins to beef up security for its nuclear facilities. The International Atomic Energy Agency issues the first international recommendations on security for nuclear material.

1973 The U.S. Atomic Energy Commission substantially strengthens its rules for security for nuclear material. John McPhee’s The Curve of Binding Energy, based on the life and ideas of weapons designer Theodore B. Taylor, makes a terrifying case that terrorists could easily steal nuclear material and make a bomb from it.

1974 India tests a “peaceful nuclear explosive,” made from plutonium from a reactor provided for peaceful purposes, touching off widespread fears over nuclear proliferation. The United States proposes an international treaty on security for nuclear stockpiles.

1975 The U.S. establishes the Nuclear Emergency Search Team (NEST) to respond to nuclear terrorist threats. The Nuclear Suppliers Group is formed; suppliers agree among other things that they will require recipients of nuclear materials to maintain at least minimum levels of nuclear security as a condition of receiving exports.

1976 President Gerald Ford indefinitely postpones plutonium reprocessing in the United States, in part over fears of nuclear theft and proliferation.

1977 The “Strengthened Safeguards Rule” imposes tough new nuclear security requirements on private owners of weapons-usable nuclear material in the United States.

1978 The U.S. Nuclear Nonproliferation Act is signed into law, imposing tough new nonproliferation requirements on U.S. exports – including a requirement to review nuclear security in every country receiving U.S. nuclear technology and materials. The CIA estimates that the most likely targets for future terrorist activity are the more than 6000 warheads stored in NATO nuclear depots in Western Europe.

1979 The Three Mile Island accident in Pennsylvania focuses public concern over nuclear energy on safety, rather than security.

1982 The Department of Energy launches an independent “Inspection and Evaluation” program to probe nuclear security weaknesses, responding to Congressional pressure over cancellation of the earlier “Independent Assessment Program,” which had identified widespread security weaknesses.

1983 President Ronald Reagan demands quarterly reports on the state of security at the Department of Energy’s nuclear facilities. The bombing of Marine barracks in Beirut provokes a new round of concern over security for U.S. nuclear facilities.

1986 The Chernobyl catastrophe again focuses international concern on nuclear safety – but also reveals what the result of a successful nuclear sabotage might be.

1989 Moscow withdraws from Afghanistan, and Arab and Afghan jihadis unite under the leadership of Osama bin Laden, with the group renamed al Qaeda.

1991 The Soviet Union collapses, and concerns arise over potential leakage of some 30,000 Soviet nuclear weapons dispersed through 11 time zones in Russia and several former Soviet republics. The U.S. Cooperative Threat Reduction program started to dismantle and secure the former Soviet nuclear arsenal. Inspections after the first Gulf War reveal that Iraq had a massive secret nuclear weapons program underway, later dismantled under international inspections.

1992 The first well-documented theft of potential nuclear bomb material is discovered – 1.5 kilograms of 90% enriched HEU from the Luch Production Association in Podolsk, Russia.

1993 Multiple thefts of HEU occur in Russia; U.S. intelligence learns of Bin Laden’s efforts to acquire nuclear materials or warheads from former Soviet republics; World Trade Center bombing occurs in New York.

1994 Nuclear thefts make headlines around the world, with 600 grams of plutonium arriving in Munich on a plane from Moscow, and nearly four kilograms of HEU found in a parked car in Prague. The first North Korean nuclear crisis – provoked when North Korea unloaded plutonium-bearing fuel from its reactor and threatened to reprocess it – ends with the negotiation of the Agreed Framework, temporarily freezing North Korea’s plutonium production.

1995 Aum Shinrikyo nerve gas attack in the Tokyo subway occurs, with investigations revealing that the group was actively seeking nuclear weapons in the former Soviet Union and elsewhere. The Oklahoma City bombing takes place.

1996 The CIA’s bin Laden unit reports on al Qaeda’s “very professional” attempts to acquire nuclear weapons.

1998 Financial crisis in Russia leads to a crisis at many nuclear facilities, with employees unpaid for months, alarms shut off when facilities fail to pay their electricity bills, and cases of guards leaving their posts to forage for food. Russian security services announce they have foiled an insider conspiracy to steal 18.5 kilograms of HEU. Al Qaeda launches simultaneous attacks on two U.S. embassies in Africa. India and Pakistan test nuclear weapons.

1999 Vladimir Putin comes to power in Russia, and strengthens the role of security services; the second phase of the Chechen war begins.

2001 September 11 attacks lead to efforts to accelerate work with Russia to secure nuclear material; establishment of the International Atomic Energy Agency’s Office of Nuclear Security, with a mandate to help states prevent nuclear terrorism; improvements in nuclear security, including tightened rules, in various other countries around the world—but no global standards set for nuclear security or actions to help or pressure countries to put security measures in place. Sultan Bashiruddin Mahmoud and another senior Pakistani nuclear scientist meet with Osama bin Laden and discuss nuclear weapons. The commander of the force that guards Russia’s nuclear weapons reveals that terrorist teams have carried out reconnaissance at two secret nuclear weapon storage facilities.

2002 An opposition group reveals a large uranium enrichment facility under construction in Iran, provoking international concern over whether Iran’s nuclear program is focused on acquiring nuclear weapons – and whether Iran might someday transfer them to terrorists it sponsors.

2003 A U.S.-led coalition invades Iraq and overthrows Saddam Hussein, in a war whose public justification hinged on the potential danger that Saddam Hussein might provide weapons of mass destruction to terrorists. After the war, CIA inspectors confirmed that Saddam Hussein had destroyed his weapons of mass destruction under the pressure of international inspection after the first Gulf War. The interception of a ship bound for Tripoli laden with uranium enrichment centrifuges helps blow the cover on a global black-market nuclear network, led by A.Q. Khan, father of Pakistan's nuclear bomb, which had been marketing centrifuges and other nuclear technology to Iran, North Korea, Libya, and possibly others for decades. By year's end, Libya agrees to abandon its weapons of mass destruction programs. North Korea kicks out international inspectors, pulls out of the Nonproliferation Treaty, and says that it has reprocessed the plutonium, raising fears that Pyongyang might now believe it has enough material for its own deterrent with enough left over to sell to others. A radical Saudi cleric issues a fatwa, or religious ruling, authorizing al Qaeda to use nuclear weapons against U.S. civilians. A court case in Russia reveals that a Russian businessman has been offering $750,000 for stolen weapon-grade plutonium for sale to a foreign client.

2004 President George W. Bush and challenger John Kerry agree that nuclear terrorism is the greatest national security threat to the United States. Global Threat Reduction Initiative is established, with a mandate to clean out potential nuclear bomb material from vulnerable sites around the world; U.N. Security Council Resolution 1540 legally requires all countries to provide "appropriate effective" security for nuclear stockpiles, although there has as yet been little substantial follow-through to implement this requirement.

2005 President George W. Bush and Russian President Vladimir Putin commit to accelerated nuclear security cooperation at their Bratislava summit, establish high-level committee to follow up, and make twice-yearly reports on progress.

2006 President Bush and President Putin announce the Global Initiative to Combat Nuclear Terrorism at G-8 summit in St. Petersburg, to “prevent the acquisition, transport, or use by terrorists of nuclear materials and radioactive substances or improvised explosive devices using such materials, as well as hostile actions against nuclear facilities.”