The rules handling multiple interacting attack and block modifications
  have evolved throughout the game's history.  Rule 500 defined legal
  attacks and blocks, here is a quick summary of the current state of
  affairs:

  When you declare an attack, you may pay any costs that are needed to
  allow creatures to attack, then pick a set of untapped creatures as
  the attacking creatures.  Check for any restrictions and requirements
  that are violated - if there are, you have to choose a different
  set of attackers.  To perform this check, first make sure that all
  of the restrictions are being met, then look at the requirements.
  If the maximum possible number of requirements are being followed,
  then the set of attackers is legal.

  Example:

  Player A controls:

  Crazed Goblin  {R}  Creature - Goblin Warrior  1/1
/ Crazed Goblin attacks each turn if able.

  Grizzly Bears  {1}{G}  Creature - Bear  2/2

  Player B controls:

  Silent Arbiter  {4}  Artifact Creature  1/5
/ No more than one creature can attack each combat.
/ No more than one creature can block each combat.

  This situation has one requirement, the Crazed Goblin must attack.
  It also has one restriction, at most one creature can attack.

  The possible sets of attackers that Player A may declare are:

    Just the Crazed Goblin: legal, as it satisfies the requirement
    without violating the restriction.

    The Crazed Goblin and the Grizzly Bears: illegal, as it does not
    satisfy the restriction.

    Just the Grizzly Bears: illegal, as attacking with just the Crazed
    Goblin is legal and satisfies more requirements.

    No creatures: illegal, as attacking with just the Crazed Goblin is
    legal and satisfies more requirements.

  The situation is analogous with blockers: all of the restrictions
  must be met, and then a maximum number of requirements must also be
  met.  An example:

  Player A controls:

  Razorgrass Screen  {1}  Artifact Creature - Wall  2/1 
/ Defender
/ Razorgrass Screen blocks each turn if able.

  Grizzly Bears  {1}{G}  Creature - Bear  2/2

  Player B controls:

  Goblin War Drums  {2}{R}  Enchantment 
/ Each creature you control can't be blocked except by two or more
  creatures.

  Player B is attacking with one creature.  Player A's blocking options
  are:

    Both Razorgrass Screen and Grizzly Bears: legal, as the requirement
    for the Razorgrass Screen to block is met, and the restriction for
    at least two creatures to block the attacker is also met.

    Just the Razorgrass Screen: illegal; while the requirement is met,
    the restriction is not, and another legal block is available that
    meets the requirements and also satisfies the restrictions.

    Just the Grizzly Bears: illegal, as the requirement is not met, and
    another legal block is available that does meet the requirement.

    Neither creature: illegal, as there is a restriction that could be
    met and the requirements can be met as well.

Some parts © 2009 Advameg, Inc.