weapons has shifted to the prevention of localized nuclear conflicts resulting from nuclear proliferation, and the threat of nuclear terrorism. However, the threat of nuclear war is considered to have resurged after the 2022 Russian invasion of Ukraine, particularly with regard to Russian threats to use nuclear weapons during the invasion. Since 1947, the Doomsday Clock of the Bulletin of the Atomic Scientists has visualized how close the world is to a nuclear war. The Doomsday Clock reached high points in 1953, when the Clock was set to two minutes until midnight after the U.S. and the Soviet Union began testing hydrogen bombs, and in 2018, following the failure of world leaders to address tensions relating to nuclear weapons and climate change issues. Since 2023, the Clock has been set at 90 seconds to midnight, the closest it's ever been. The most recent advance of the Clock's time setting was largely attributed to the risk of nuclear escalation that arose from the 2022 Russian invasion of Ukraine. Types of nuclear warfare The possibility of using nuclear weapons in war is usually divided into two subgroups, each with different effects and potentially fought with different types of nuclear armaments. The first, a limited nuclear war (sometimes attack or exchange), refers to the controlled use of nuclear weapons, whereby the implicit threat exists that a nations can still escalate their use of nuclear weapons. For example, using a small number of nuclear weapons against strictly military targets could be escalated through increasing the number of weapons used, or escalated through the selection of different targets. Limited attacks are thought to be a more credible response against attacks that do not justify all-out retaliation, such as an enemy's limited use of nuclear weapons. The second, a full-scale nuclear war, could consist of large numbers of nuclear weapons used in an attack aimed at an entire country, including military, economic, and civilian targets. Such an attack would almost certainly destroy the entire economic, social, and military infrastructure of the target nation, and would likely have a devastating effect on Earth's biosphere. Some Cold War strategists such as Henry Kissinger argued that a limited nuclear war could be possible between two heavily armed superpowers (such as the United States and the Soviet Union). Some predict, however, that a limited war could potentially "escalate" into a full-scale nuclear war. Others have called limited nuclear war "global