What is Wizard's War?
Wizard's War is the ultimate conflict between two magi. Hermetic Law allows Wizard's War to be declared for any reason, and there can be no Tribunal sanction against a magus for killing or otherwise harming his foe during a justly declared and executed Wizard's War.
A formal Wizard's War is initiated by the delivery of a declaration (usually via a

Redcap messenger) on the night of the full moon. The Wizard's War itself begins one month later, on the night of the next full moon, and lasts for one month (until the rise of the next full moon after that).
Wizard's War is usually fought for political gain, to settle personal disputes, or to capture resources. Sometimes a magus declares Wizard's War merely to force his opponent to cower within the sanctuary of his covenant, rather than as a serious attempt to cause direct harm. This might occur when one magus is wanting to prevent another weaker magus from attending some event, such as a Tribunal or a Mystery Cult activity like the Gathering of Twelve Years. Declaring Wizard's War for these reasons can be risky, as the supposedly weak target of a Wizard's War may have unexpected allies who could counter-declare against the aggressor magus.
As declarations of Wizard's War must be delivered on the night of a full moon, the earliest that counter-declarations can be made in response to a Wizard's War is one month later, on the eve of the original War, and these declarations will not have legal effect until the close of the first Wizard's War.
Magi often use the month of grace after receiving a declaration of Wizard's War to organize their defenses. There is not usually enough time for a magus to research or learn any new spells, so often his preparation consists of traveling to a place of safety (perhaps one of the major covenants of Mythic Europe which are protected by high magnitude Aegis of the Hearth rituals), gathering vis, and casting defensive spells that he already knows. A particularly devious magus might try to prevent his foe from preparing, by surreptitiously attacking and killing his foe prior to the declaration of Wizard's War. Necromantic magic can then be used to keep the death from being discovered until the legitimate Wizard's War period. Clearly, such a plan would be a Hermetic crime, but it is one that a well-prepared magus might feel he can get away with, particularly if his foe has a reputation for reclusively working long hours in his laboratory.
Sometimes an individual declares Wizard's War on many individuals, such as everyone living at a particular covenant. This strategy prevents a magus from being prosecuting for causing damage to the persons or property of the sodales of the true Wizard's War target. Groups of magi might also declare Wizard's War on an individual or another group, such as another covenant. This allows the legal use of spells such as Wizard's Communion when casting Intangible Tunnel-style effects. If an entire covenant of magi declares Wizard's War on an individual or another covenant, it is a good idea for one of the magi to refrain from declaring Wizard's War on their enemies, and remain uninvolved. This tactic offers the aggressors' own covenant some legal protection from counter attacks during the Wizard's War, as the uninvolved magus' life or property cannot legally be damaged.
More general information about Wizard's War can be found in Houses of Hermes: Societates, pages 22–25, and Houses of Hermes: True Lineages, pages 47–48.

The essential ingredient for waging Wizard's War in this manner is the spell Opening the Intangible Tunnel (ArM5, page 162). A few variations on this spell, and ways to manipulate the tunnels, are described here.