Sentence:  On the one hand , the major European nations had to maintain vis-a-vis each other an emphasis upon sovereignty , independence , formal equality - thus insuring for themselves individually an optimal freedom of action to maintain the `` flexibility of alignment '' that the system <WSD>required</WSD> and to avoid anything approaching a repetition of the disastrous Napoleonic experience . 
Meaning of the word marked with <WSD> tags:  require as useful, just, or proper