Of a soldier, to get into position in a rank.(verb)
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Use "fall in" in a sentence
"But when we go on tour I will be such a nice, sweet, charming, beautiful person Joe McIntyre will fall in love with me, not love at first sight but a strong friendship that will build up to an everlasting love."
"When the Admiralty does its calculations it always thinks that winds blow forever in the right quarter—that masts never snap—that spars never come adrift—and that sails never split, fall in the slings or work loose from the reefing pendants."
"They would argue their cases vehemently and stand their ground as long as possible, but once a decision was madewhether it was to keep troops in Bosnia longer or to threaten air strikes against Belgradethey would all fall in line."