US President Joe Biden makes alarming remarks in doomsday address. This is what he said

The United States and Russia have the two largest nuclear arsenals in the world, by far. They include weapons that can be delivered by aircraft, submarine and land-based ballistic missiles. In both countries, the president has sole authority to order a nuclear strike. US officials indicated they did not know what Putin intended when he said that, AP reported.

But what does Putin mean by putting nuclear weapons on alert? According to U.S. nuclear doctrine, the weapons' alert level is central to their role in deterring attack. The idea is that being prepared to respond on short notice makes an enemy less likely to attack in the first place and risk retaliation that would do incalculable damage.

A counterargument is that having ICBMs, which the Pentagon calls the most responsive portion of its nuclear arsenal, on high alert during a crisis compresses a president's decision-making room and leaves open the possibility of ordering them launched in response to a false alarm. The 400 deployed U.S. ICBMs are armed at all times.

Some arms control experts have argued for taking ICBMs off high alert by separating the missiles from their nuclear warheads. But in a crisis, perhaps like the one implied by Putin's alert order, a decision to re-arm the missiles would be taken as an escalatory move that could make the crisis even worse. There is no evidence, however, that the Biden administration has reciprocated in any sense to Putin's announcement.