WARSAW Polish Prime Minister Donald Tusk pledged on Thursday to push ahead with a “great modernization program” for his country’s military, a day after Russian drones crossed into Poland and forced the NATO alliance to confront a potential threat in its airspace for the first time. European officials described Sept. 10’s incursion, which occurred during a wave of unrelenting Russian strikes on Ukraine, as a deliberate provocation. It deepened longstanding fears that the three-year war between