Why would an army want to flank its opponent, or attack its opponent from the side, instead of head on?

The major impact third parties have on presidential elections has been to.. a. encourage larger vote turnout b. recruiter leader from the democratic and republican party to run for president c. generate increased party identification among the electorate d. developing issues that are later adopted by the major political parties e. drawn off of votes from the major parties to throw election into the house representatives