Answer:

siege

Answer	Crossword Clue
SIEGE	Genghis Khan tactic
SIEGE	Campaign that may involve catapults and trebuchets
SIEGE	Alternative to a blitz

SIEGE	Military operation that might last for months
SIEGE	The Alamo had a famous one
SIEGE	Lengthy attack
SIEGE	Military action that includes a blockade
SIEGE	All-out attack
SIEGE	Tactic in a war of attrition
SIEGE	W.W. II ordeal at Leningrad
SIEGE	Counterpart of a blitz

SIEGE	Drawn-out campaign
SIEGE	Wait-'em-out strategy
SIEGE	Prolonged attack
SIEGE	Lengthy bout
SIEGE	Challenge for defenders
SIEGE	Military blockade
SIEGE	Long bout
SIEGE	Kind of mentality

SIEGE	You're being attacked while under it
SIEGE	Tactic for Napoleon
SIEGE	More than a raid
SIEGE	Attack tactic
SIEGE	Military operation
SIEGE	Crusades tactic
SIEGE	War tactic
SIEGE	Drawn-out fight

SIEGE	Preattack tactic
SIEGE	Long series of troubles
SIEGE	Operation at the Alamo
SIEGE	More than a 23-Across
SIEGE	Warfare tactic
SIEGE	Not just a raid
SIEGE	Start of a long battle
SIEGE	Long campaign

SIEGE	1941 Leningrad event
SIEGE	Cause of a downfall
SIEGE	Beleaguerment
SIEGE	See 63-Across