The US wants to cut its budget, but NATO is putting pressure on them to keep their commitments under Article 5.

Article 5 of the NATO treaty requires the Alliance to come to the aid of member-states if their territory comes under attack.

[…]

For the new NATO members, American troops continue to act as a hedge against what they see as Moscow’s unpredictable foreign policy.

“The nations that have been most concerned about the credibility of Article 5 are the Central Europeans,” Brzezinksi [former US Deputy Assistant Secretary of Defense for Europe and NATO Policy] said. “People remember that Estonia had a cyber attack that originated in Russia, and which most believe was organized by the Russian government.”