HQ Tracing Rules

One activity that will become second nature to the veteran EOTS player is the tracing of HQ "ranges"--a path of hexes from an HQ to a unit, supply source or battle hex. But to a new player, the differences in how different types of ranges are determined can be confusing at first. There are essentially four different "ranges" that are required for various game activites:

Supply: Used to determine if units can activate or during attrition, if out of supply units "flip"
Activation: Used to activate in supply units for Offensive or Reaction Movement
Reaction: Used to determine if a Reaction player's HQ is eligible to activate units during Reaction Movement
Attrition: Used to determine if reduced units are eliminated during the attrition phase (yes this is different than the Supply range above).

Supply Tracing rules

Example 1:The Japanese 23rd Air Flotilla in Makassar receives supply from the S HQ. The supply line combines sea and land tracing by going in and out of Japanese controlled ports on Borneo and is exactly 10 hexes, the range of the S HQ.

Example 2:The Japanese 38th Army in Rangoon is Out of Supply. A land supply line from the S HQ is blocked by the Indian Corps on the Burma border and by the China hexes. Each hex in China is considered to be "occupied" by an enemy unit. Likewise, hexes with intrinsic garrisons are considered to be enemy occupied. The supply line by sea is two hexes short given the S HQ's range of 10.

The supply rules in v2.0 of the EOTS rules are more straightforward and less ambiguous than earlier versions of the rules. However, for first time players, the description is still a challenge to grasp. This article describes the intent of the rules from a player's perspective and gives some additional examples.

Rather than think in terms of hexsides crossed, it is more intuitive to think of supply as a two step process.

Step 1: Geographic Considerations

A supply line starts at an HQ and is traced either to an ultimate supply source (to put the HQ in supply) or to a unit (to put the unit in supply if the HQ is in supply). The first consideration is that the possible paths taken by the supply line must conform to the geography of the map.

A supply line from an HQ to an ultimate supply source can be of any length.
A supply line from an HQ to unit is limited to the HQ's range in hexes.
A supply line traced from land to sea must exit the land at a friendly port
A supply line traced by sea cannot cross an unplayable Ocean hexside

Combining Sea and Land Supply Lines

A supply line can be traced "by sea" and "by land" or a combination of the two. A supply line traced by sea can supply any units in a coastal or one hex islands hex. A supply line traced by sea that enters a land mass through a friendly port can then be traced by land to any hex on the land mass (limited of course by the HQ range).

It is possible for a supply line to leave an HQ by sea, trace to a friendly port, then trace by land to another friendly port and then trace back out to sea again (sea-land-sea; a land-sea-land combination is possible as well). As long as the supply line enters and exists via friendly ports, sea and land supply lines are linked. (See Example 1.)

Step 2: Enemy unit considerations

Once you have determined whether units and HQs have met the range and geographic considerations for a supply line, you next need to determine if the presence of enemy units block the supply line.

There are two ways enemy units can block a supply line:

Supply traced by land cannot go into or through a hex occupied by only enemy ground and/or air unit(s). Supply can be traced into and through a battle hex occupied by ground/air units of both sides. (See Example 2.)
Supply traced by sea cannot go into or through a hex with an unneutralized enemy air Zone of Influence.

Determining whether air units neutralize an enemy air ZoI can be tricky since air units that are out of supply do not possess a ZoI. I recommend the following procedure:

Assume that each air unit has a ZoI if it has a potential supply line after considering the HQ range, geography and enemy presence for the land portion of the supply line (if any).
Opposing air units that now mutually negate each other's ZoI that would otherwise block sea supply to each other, are in supply. Air units whose supply line avoids enemy ZoI altogether are also in supply.
Opposing air units that would put each other out of supply due to unneutralized ZoI fall under a special case. In these situations, the Japanese units will be out of supply.

Example 3: The Twilight Zone Effect In the above picture, the only HQ in range of the Japanese air unit is S HQ in Saigon (i.e., the supply line must go through Miri and Balikpapan) and the only HQ in range of the US air unit is ABDA. If the Japanese air unit is in supply, then its ZoI in Kendari would block supply to the US air unit. But if the US air unit is in supply, then its ZoI in Miri would block supply to the Japanese air unit. This infinite loop is solved by favoring the Allies and in this example the US air unit retains its supply, cutting supply to the Japanese air unit.

One other note about the geographic considerations: In some cases the shortest distance between two points will meet the geographic considerations of a supply line but will be blocked by enemy units. Range permitting, it might be possible in these situations to take a longer path that avoids the blocking units or ZoI. (Once you start playing, this point will become obvious.)

The examples in the rulebook are fairly comprehensive (though not meant to be realistic game situations per se) so I recommend that you use them to further understand possible supply situations. Note that in Example 2, the supply path to Unit D would need to go through Soerabaja. (The example shows the supply line being traced across a hexside with unplayable ocean/greyed out land which is not allowed.)