The United States moved after Russian President Vladimir Putin announced on Friday the largest annexation operation in Europe since World War II, after Russia controlled 15% of Ukraine, according to Reuters.

The United States quickly imposed comprehensive sanctions on Russia due to its announcement of annexing a part of Ukraine, targeting hundreds of individuals and companies, including those in the Russian military-industrial complex and members of the legislature.

Janet Yellen, the US Treasury Secretary, said in a statement, "We will not stand idly by while Putin fraudulently attempts to annex parts of Ukraine," and Antony Blinken, the US Secretary of State, explained in a statement that "the United States unequivocally rejects Russia’s fraudulent attempt to change Ukraine’s internationally recognized borders, including holding sham referenda."

The US Treasury Department emphasized that it imposed sanctions on 14 people in the Russian military-industrial complex, two leaders of the central bank in the country, family members of senior officials, and 278 members of the Russian legislature "to enable Russia’s sham referenda and attempt to annex sovereign Ukrainian territories."

The Treasury Department also issued a guidance warning about increased risk of sanctions on those outside Russia if they provide political or economic support to Moscow.

The US State Department said in a separate statement that it imposed visa restrictions on more than 900 people, including members of the Russian and Belarusian military and "Russia’s agents for violating Ukraine’s sovereignty, territorial integrity, and political independence," prohibiting them from traveling to the United States.

Do the sanctions imposed by the United States hurt Russia?

Sanctions are considered a powerful and unique foreign policy tool in the hands of Americans because a large part of the global economy is executed in US dollars and ultimately is subject in some way to US law.

David Feith, chief economist at Argus Media, said sanctions weaken Russia in other ways, "especially by blocking its access to high-tech components needed by its military sector," according to his statements to the BBC.

The sanctions imposed by the United States and its European allies on Russia, which now include a US ban on Russian energy imports, are the most restrictive ever against one of the world's largest economic powers.

The measures, which include sanctions targeting the Russian financial system, the wealth of influential individuals, and Russian fossil fuels, aim to punish Vladimir Putin and the ruling minority who support and rely on him, and to undermine the Russian economy.

This means a massive economic slowdown, as the Russian stock market has been closed since the invasion, many Russian companies with shares listed abroad have seen their shares almost completely fall, and the Russian ruble, which collapsed at the start of the war and sanctions, is near its all-time low.