There are five Defcon alert levels, where number one is the highest, which means a state of war or nuclear threat.
"The current Defcon level overall is now at defcon 3 today as there is a raised CENTCOM [US Central Command] due to rocket attacks against U.S. bases and airstrikes carried out by the U.S. military and a raised EUCOM [US European Command] level due to the United States activating an artillery command in Europe as Russia amasses troops and artillery near the Ukrainian border and raised AFRICOM [US Africa Command] level due to ongoing conflict in Ethiopia," the Defcon Level reported.
Defcon 3 was reached during the 1991 Gulf War, partially during the 1962 Cuban missile crisis, and most recently after the 9/11 attacks in 2001.
In the past few months, Western countries and Ukraine have accused Russia of deploying additional troops along the Ukrainian border in alleged preparation for invasion. Moscow has repeatedly dismissed these allegations, saying that it has no intention of invading Ukraine, while stressing that it has the right to move its troops within its territory.
Russia has also expressed concerns over NATO military activity near its borders and ongoing military support of Ukraine, including an increase in the number of Western military advisers in the breakaway Donbass region and increased arms deliveries to Kiev.