Ukraine hit Russia in parts of Russia they couldn't hit before with U.S.- and UK-supplied missiles that may have required NATO personnel to aim and/or fire. That's an escalation -- hasn't been done before, brings NATO countries more directly into the war.

Russia hit parts of Ukraine they could already hit, but with a much more advanced type of missile that can carry a nuclear payload. That's not an escalation -- Russia could already strike there! -- but it is a reminder about what further escalation could bring.