Leaders are "units" but they are not "land units" (p. 6-7).

Avoid Battle is only allowed "when a power enters a space containing a stack of enemy land units," and "some or all of those enemy units" may evade (p. 18).

Two questions have come up in the game we're playing:

1) If a stack enters a space containing only an enemy leader, can that leader attempt to evade? It would seem to me that by the text of the rules the answer is clearly no, nonsensical as this may be from a reality standpoint.

2) If a stack enters a space containing both enemy land units and an enemy leader, can the enemy leader evade by himself (without taking any land units with him)? This seems ambiguous. The phrase "those enemy units" may refer to the "land units" mentioned in the previous sentence, or simply to any enemy units in the space.

1) The rules on movement state:

"One or more army leaders may move without accompanying land
units as long as they don’t enter a space controlled by an enemy
power or containing enemy units. If an army leader is ever alone
in an unfortified space when enemy land units enter due to enemy
movement, retreat, or interception, that leader is captured.
Place the captured leader on the enemy power card. He may be
regained in the Diplomacy Phase of an upcoming turn (see Section
9)."

So a leader alone in a space is captured without any change to avoid battle.

2) The rules on avoid battle include this in Step 3 of the Avoid Battle Procedure:

"Choose Units: The power chooses which land units and army
leaders will attempt to avoid battle. The total number of units chosen
may exceed the number that can be moved as a single formation.
Any number of units may be left out of the avoid battle attempt."

Since "any number" may be left out, that includes the case of a leader alone trying to avoid the battle. So this is legal ... though his soldiers may not have much respect for their leader afterwards!

Thanks as always, Ed!