Digging-in when defending a position was a standard practice by the start of WWI. To attack frontally was to court crippling losses, so an outflanking operation was the preferred method of attack against an entrenched enemy. After the Battle of the Aisne in September 1914, an extended series of attempted flanking moves, and matching extensions to the fortified defensive lines, developed into the "race to the sea", by the end of which German and Allied armies had produced a matched pair of trench lines from the Swiss border in the south to the North Sea coast of Belgium.