Wikipedia reports it like this:
Keep Calm and Carry On was a motivational poster produced by the British government in 1939 in preparation for the Second World War. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities. Although 2.45 million copies were printed, and although the Blitz happened, the poster was hardly ever publicly displayed and was little known until a copy was rediscovered in 2000.
Between September of 1940 and May of 1941 London was bombed 71 times. 100 tons of high explosives were dropped on cities in England. One million houses in London were either destroyed or damaged and more than 40,000 citizens of England were killed. War is bloody and dangerous.
2 Chronicles reports it like this:
And when they began to sing and praise, the LORD set an ambush against the men of Ammon, Moab, and Mount Seir, who had come against Judah, so that they were routed. For the men of Ammon and Moab rose against the inhabitants of Mount Seir, devoting them to destruction, and when they had made an end of the inhabitants of Seir, they all helped to destroy one another. (2 Chronicles 20:22-23 ESV)
Three enemies are storming Jerusalem. All conventional wisdom says do anything but sing. Plan your strategy. Draw your swords. Man your stations. Certainly there are times when conventional wisdom works. Then there are times when conventional wisdom falls short, when you’re outnumbered, when the enemy is just too strong and the odds are stacked against you.
What can you do in those times?
Keep calm and pray. Remember Jehoshaphat’s prayer? We don’t know what to do but our eyes are on you. Jehoshaphat prayed God’s character and remembered God’s works.
Keep calm and worship. He did the most unconventional thing: he placed the worship leaders in front of the generals, the praise team in front of the army. The choir marched into battle first. You see, we have the advantage of knowing the outcome. Jehoshaphat didn’t. He had the diagnosis, not the prognosis.
I love the progression of events! And when they began to sing and pray, the Lord. Their sacrifice of praise filled the halls of heaven. God acted on their behalf. He created confusion in the enemy camp, turned them on one another, and his people watched it unfold.
Whatever you’re facing today, sing. I know it sounds simple, maybe even trite. It isn’t. Worship in spite of your circumstances. Do not allow your current predicament to rob you of giving God the praise He deserves. Do not allow your current crisis to curtail your worship of the crucified and risen Christ.
Keep calm and worship.