A unit that is on the march is not prepared for combat, so troops are reluctant to march whilst a potentially threatening enemy is nearby. If you wish a unit to march when a non-fleeing enemy unit is within 8", it will first have to take a Leadership test. If the test is passed, the unit disregards the nearby foe and marches as ordered. If the test is failed, the unit refuses to march and will only be able to move normally. Note that if a unit attempts at this test and fails, it is still treated as having marched, even if its controlling player then elects not to have the unit move at all.