Has any nuclear-protected country been under any significant attack by another country's military forces?

Countries can have nuclear weapons of their own (e.g. Russia, US) or be protected by them through alliances (e.g. Baltic states via NATO, Belarus via Collective Security Treaty Organization, Warsaw pact countries under USSR).

While there have been accidents, brief incursions, and minor skirmishes involving nuclear-protected countries, are there any examples of significant military engagements against a nuclear-protected country?