Master Warcraft and Fork

[LotusHead]

My opponent is gonna beat down on me for lethal damage with Master Warcraft on the stack.  My only recourse is to Fork it.

I understand that my copy resolves first (I get to chose how creatures attack and block), then my opponents original copy resolves (He gets to chose how creatures attack and block).  Am I screwed?

[Joblin Velder]

Totally.

[Khahan]

My opponent is gonna beat down on me for lethal damage with Master Warcraft on the stack.  My only recourse is to Fork it.

I understand that my copy resolves first (I get to chose how creatures attack and block), then my opponents original copy resolves (He gets to chose how creatures attack and block).  Am I screwed?

Both players are attempting to make choices at the same time.
#  103.4 - If both players would take an action at the same time, the active player (the player whose turn it is) makes any choices required, then the nonactive player makes any choices required, then the actions happen simultaneously. This rule is often referred to as the "Active Player, Nonactive Player (APNAP) rule." [CompRules 2003/07/01]
Example: A card reads "Each player sacrifices a creature." First, the active player chooses a creature he or she controls. Then the nonactive player chooses a creature he or she controls. Then both creatures are sacrificed simultaneously. [CompRules 2003/12/01]

So, since your opponent is active player, he chooses first.  Then you get to choose. The attacking creatures and blocking creatures are not declared attackers or blockers until all steps in rule 308.2, Declare attackers step is complete.  Remember, Master Warcraft only lets you choose which, if any, creatures attack or block. It does not let you make decisions about paying costs for those creatures.  It only affects 308.2a, it does not affect any other portion of declaring attackers (or 309.2a for blockers). A creature is not officially an attacker/blocker until those steps are complete.

The first step you do is choose attackers (and it is this action and this action only that is getting replaced by master warcraft. Then you tap creatures, then you pay all costs.
Once these steps are complete, then and only then is a creature declared as an attacker.

So, if I am active player and I announce my Darksteel colossus and my grizzly bear as an attacker that is all I do at this point. You now get to decide which creatures are attacking and which are not. You decide the colossus is not attacking but the bears is.

At this point, master warcrafts effect is done for attackers. Its time to tap creatures and pay any costs (for example, if ghostly prison was in play).

Both players pass priority and its time to declare blockers (following the steps in 309.2)  Your opponent announces no creatures as a blocker. However, you decide that your white knight is a blocker.

You both pass priority and move onto combat damage.

In short, you get lucky that one turn.

[LotusHead]

Master Warcraft
Instant
Oracle text:
Play Master Warcraft only before attackers are declared.
You choose which creatures attack this turn. You choose how each creature blocks this turn.

Thanks for the more in-depth analysis.

I was actually wondering if the ability of Master Warcraft simply replaces the previous ability.

ie: (since I am being beat down and Forked the spell)  When my Fork copy resolves, I am now given the power to decide if and how creatures attack and I decide if and how creatures block.  When my opponent's original Master Warcraft resolves, My opponent decides how creatures attack (as is usual) and my opponnet decides how creatures block.  That is simply the stack resolving.  Then my opponent announces the start of the attack step and the headaches begin.

For clarity, My opponent wants to attack with his fatty creature (5/2 Boros Swiftblade in reality) and if that creature DOES attack, I would very much like to block with my chump blocker.  My hand was Fork and 2 irrelevant cards, and I was at 10 life.

1) It seems my opponent's Master Warcraft replaces my Fork of Master Warcraft as far as who decides what when.  Or do we "both" get to decide if creatures attack or block using APNAP?

The example cited (each player sacs a creature) is resolving an ability that both players have to do, and only explains what order they happen.

2) Are replacement abilities always noted by "if something would" for easy identification? (like Triggered and Activated abilities have easy to recognize keywords/templates)

[Matt]

Replacement abilities use the"If/instead" construction, though it isn't, strictly speaking, a template.

IF [card]Darksteel Colossus[/card] would be put into a graveyard from anywhere,
reveal Darksteel Colossus and shuffle it into its owner’s library INSTEAD.

[card]Abundance[/card]:
IF you would draw a card, you may INSTEAD choose land or nonland and reveal cards from the top of your library until you reveal a card of the chosen kind. Put that card into your hand and put all other cards revealed this way on the bottom of your library in any order.

Do note that it's the paired use of the words "If" and "Instead" that denotes a replacement ability. [card]Cabal Ritual[/card], for example, has no replacement abilities, though it does use the word "instead":

Add to your mana pool.
Threshold — Instead add to your mana pool.

[Khahan]

I was actually wondering if the ability of Master Warcraft simply replaces the previous ability.

ie: (since I am being beat down and Forked the spell)  When my Fork copy resolves, I am now given the power to decide if and how creatures attack and I decide if and how creatures block.  When my opponent's original Master Warcraft resolves, My opponent decides how creatures attack (as is usual) and my opponnet decides how creatures block.  That is simply the stack resolving.  Then my opponent announces the start of the attack step and the headaches begin.

A couple of quick notes:
Master warcraft is not a replacement effect. It is a control effect similar to [card]mindslaver[/card].

Also, you do not make any choices as to attackers and blockers when the spell resolves. You make the choices when you normally would, during the declare attackers and declare blockers steps. 

For clarity, My opponent wants to attack with his fatty creature (5/2 Boros Swiftblade in reality) and if that creature DOES attack, I would very much like to block with my chump blocker.  My hand was Fork and 2 irrelevant cards, and I was at 10 life.

1) It seems my opponent's Master Warcraft replaces my Fork of Master Warcraft as far as who decides what when.  Or do we "both" get to decide if creatures attack or block using APNAP?

You both get to decide using the APNAP rule as detailed above. Again, there is no replacement effect with Master Warcraft.

The example cited (each player sacs a creature) is resolving an ability that both players have to do, and only explains what order they happen.

The example cited is an illustration of 1 way to apply the rule. The rule is what you should be paying attention to and that states that you follow the APNAP rule because both players are attempting to make a choice at the same time.
The example is actually in the rules as an example. They cannot give examples for all possible scenarios this rule covers or else the rulebook would be long...err longer...err

[Godder]

Questions involving more than one Master Warcraft resolving in a turn have come up in Ask The Judge, and the basic answer is that the player to resolve the most recent Master Warcraft (or copy) in a given turn is the one who gets to make the decisions.