The phrase '___ If By Land...' is a reference to the famous Longfellow poem, 'Paul Revere's Ride'. The poem tells the story of Paul Revere's midnight ride to warn the American colonists of the British army's approach. The phrase is used to refer to a warning or alert of some kind. The crossword solution for this phrase is 'SOS', which stands for 'Save Our Souls' and is an international distress signal used to call for help in times of emergency.