A March is a single player's squad sent to attack or gather. A Rally is multiple players' marches combined into one coordinated attack, led by whoever starts the rally - this is how alliances take down targets far stronger than any one member could handle alone.
When a rally is led, the RALLY LEADER's relevant stats (including things like Virus Resistance during seasons where that matters) apply to the whole group's effectiveness, not each individual member's own stats. This is why many alliances designate a strong, well-invested member to lead rallies against tough targets - it lets less-developed members meaningfully contribute to fights they couldn't take on solo.
Morale, hospital capacity, and healing time all matter in combat: losing units sends them to your Hospital rather than eliminating them permanently (up to your hospital's capacity), so building enough hospital space is a real strategic consideration, not just a side stat. Once your hospital caps out, further losses can become permanent, so don't neglect this building while chasing pure offense stats.
Before attacking anything - a monster, a rally target, or another player - check troop type matchups and relative power first. A fight that looks winnable on raw Power alone can still go badly if you're the disadvantaged troop type in the counter triangle.