Complex sentence: In late December 1916, reports from witnesses, led to British and French air reconnaissance sorties further to the south and in mid-January 1917, British intelligence concluded that a new line was being built from Arras to Laon.
Split into simpler sentences: In late December 1916 reports from witnesses, led to British and French air reconnaissance further to the south. In mid-January 1917, British intelligence concluded that a new line was being built from Arras to Laon.