Proverbs 21:31 (ESV) … “The horse is made ready for the day of battle, but the victory belongs to the Lord.”
A nation’s best line of defense is an active trust in God. History abounds with illustrations of this truth.
Consider, for instance, the evacuation of the British army from Dunkirk. Everyone agrees it was a miracle. The bulk of the expeditionary force, which the government feared would be lost, was saved by an armada of tiny vessels that plied the English channel constantly against enormous odds and with remarkably few casualties. The evacuation was possible only because of the weather—and God alone controls the weather.
Lord Gort, who was in command of the British army at Dunkirk, was given permission to capitulate if he thought it best to do so. Within forty-eight hours after he received this authority from the British war office, however, light began to pierce the dark clouds. When the dikes were opened to flood the low ground to delay the German advance, the wind blew in from the sea, greatly aiding this strategy. And during five out of the seven days and nights of the evacuation, the wind blew from the land. Without these offshore winds, embarkation in small boats from that dangerous coast would have been impossible.
On June 4, 1940, the British prime minister told the House of Commons: “A fog did its helpful work in screening from the innumerable German aircraft the motley mass of vessels sent to rescue the men; and the swift current of the English Channel gave way to calm waters—a happening almost without precedent at this time of year.”
General Sir Beauvoir De Lisle declared at the central hall of Westminster on November 2, 1943, “I believe this war was won on May 26, 1940, the Sunday the King appointed as a Day for National Prayer.”
Weather also prevented a German invasion of Britain. The Germans intended to invade England during the weekend of September 16, 1940, because that was the week of the full moon and the harvest moon, when the waters of the English channel are generally smoothest. In fact September 16–20 is the period chosen for the yearly attempts to swim the straits of Dover because the harvest moon prolongs light to a late hour and the weather is usually calm. Having full knowledge of the facts, Churchill told Parliament and the nation that the next ten days would be the most critical in the country’s history.
The Germans, who had collected boats at various points on the French coast, waited for the critical time. The weather remained calm right up until September 16 and it seemed that the Germans would have their way. Then on September 17 a gale blew up the channel and the sea became rough. The bad weather lasted until September 29! The Germans could not use their invasion boats and these were soon systematically destroyed by the British air force. Once again, the previous Sunday had been kept as a Day of National Prayer.
The Germans, persistent as ever, set two new target dates. They planned to try again in November or December, when fog usually blanketed the channel. However for the first time in living memory there was no fog that winter in the straits.
Then the Germans set February 15, 1941, as another invasion date. On February 14, however, a submarine earthquake and a hurricane occurred in the Atlantic. The effects of the resulting tidal system around the British Isles were both immediate and prolonged. Ships were driven as far as eighty miles off course. The Germans gave up. Truly, “safety is of the Lord.” 
 Phillips, J. (2009). Exploring Proverbs 19–31: An Expository Commentary (Vol. 2, Pr 21:30–31). Kregel Publications; WORDsearch Corp.