In May 1998, two weeks after India detonated five nuclear devices, Pakistan responded by conducting five of its own nuclear tests. Both countries claim to need nuclear weaponry to protect themselves from their nuclear-armed neighbor.
 "Today God has given us the power in order to save our kingdom from danger," said Pakistani Prime Minister Nawaz Sharif at the time. "It was the final solution, which we had to do. In 1974, when India first carried out an atom bomb explosion, we did not have all the know-how then. This nuclear test saved us from a new danger."
India said the Pakistani tests proved the need for its own and the United States condemned the response, then-President Clinton saying he "deplored" the move.
"By failing to exercise restraint in responding to the Indian tests, Pakistan lost a truly priceless opportunity to strengthen its own security, to improve its political standing in the eyes of the world," he said. "And although Pakistan was not the first to test, two wrongs don't make a right."
He imposed sanctions on Pakistan, which added to those imposed by the first Bush administration after receiving intelligence Pakistan was working on building a nuclear bomb. The United States also placed sanctions on India.
The two nuclear-armed countries have been testing missiles since then, even as they engage in peace talks over the disputed Kashmir region.
In December 2004, Pakistan tested a surface-to-surface ballistic missile capable of hitting targets deep inside India. Later that month, India successfully hit a test target with a new surface-to-surface missile with a range of 180 miles, Reuters reported.
Pakistan's program
Pakistan began its nuclear program in 1972 under Minister for Fuel, Power and Natural Resources Zulfiqar Ali Bhutto, who later became president and prime minister.
India's first test of a nuclear device in 1974 caused Pakistan to scramble to acquire uranium enrichment technology and expertise to advance its own nuclear program.
The arrival of German-trained metallurgist Dr. Abdul Qadeer Khan in 1976 sped up those efforts with his knowledge of gas centrifuge technologies picked up through his work at the classified URENCO uranium enrichment plant in the Netherlands, according to the Federation of American Scientists. He also reportedly brought with him stolen uranium enrichment technologies from Europe.
Khan was placed in charge of building, equipping and running Pakistan's Kahuta facility, established in 1976.
By 1986, Pakistan is thought to have acquired enough fissile material for a nuclear weapon, and a year later was able to carry out a nuclear explosion, according to Pakistani sources, the FAS reported.
On May 28, 1998, two weeks after India resumed nuclear testing, Pakistan announced it had tested five nuclear devices -- a boosted fission device and four sub-kiloton nuclear devices.
Two days later the nation claimed to have tested one more nuclear warhead with a yield of 12 kilotons, however, the detonations could not be confirmed through seismic means.
The Natural Resources Defense Council estimates that Pakistan has built 24 to 48 highly enriched uranium-based nuclear warheads, but Pakistani officials claim their nuclear weapons are not assembled. The fissile cores are stored separately from the non-nuclear explosives packages, they say, and that the warheads are stored separately from the delivery systems.
The U.S. Defense Department in 2001, however, reported that India and Pakistan have the ability to assemble nuclear weapons fairly quickly.
The terrorist attacks on Sept. 11, 2001, raised concerns about the security of Pakistan's nuclear arsenal, and two days later, Pakistan reportedly moved nuclear weapons components to six secret locations. Gen. Pervez Musharraf then fired his intelligence chief and other officers and detained several suspected retired nuclear weapons scientists, in an attempt to root out extremists, according to the FAS.
Concerns have been raised about Pakistan leaking nuclear materials and expertise since then. In November 2002, shortly after North Korea admitted to pursuing a nuclear weapons program, the press reported allegations that Pakistan had provided assistance in the development of its uranium enrichment program in exchange for North Korean missile technologies, according to the Federation of American Scientists.
When Libya in December 2003 made the surprise announcement that it had been working on weapons of mass destruction programs and would voluntarily abandon them, officials from the country also said they purchased nuclear components from the black market, including from Pakistani scientists.
The revelations confirmed American and British intelligence findings that A.Q. Khan, the father of the Pakistani nuclear program, was running a secret network distributing nuclear weapons information to other countries. Khan later admitted on Pakistani television to handing over nuclear information to Libya, Iran and North Korea without authorization. For more on Khan, go to his profile.
Like India, Pakistan has not signed the nuclear Non-Proliferation Treaty or the Comprehensive Test Ban Treaty.
-- Compiled by Larisa Epatko for the Online NewsHour
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