It's debatable: funding and supporting a proxy since 2023 was a pretty effective way to keep Russia into a quagmire for years, seriously affecting their military, draining their finances, and isolating their economy - to the point that any non-military activity outside of the country has basically disappeared. Short of direct engagement, which is unfeasible due to MAD, it basically was the strongest response.

(2014 is different, in the sense that it was always going to happen if Ukraine ever managed to escape Russian orbit, and more or less everyone knew.)