Intrusion
An intrusion is a move that threatens to disconnect a virtual connection of the opponent. The most common kind of intrusion is a template intrusion, which is a move inside the carrier of one of the opponent's templates.
By definition, a template or a virtual connection can always be defended, and therefore, intruding into a template does not actually disconnect the opponent (unless the opponent declines to defend the template). Nevertheless, a well-played intrusion can be to the benefit of the intruding player. For example, the intruding player may gain a bit of territory or a ladder escape.
Choosing which one of several possible intrusions is the most advantageous is one of the most difficult skills in Hex.
Contents
Examples
Bridge intrusion
The most basic template is the bridge. It has two potential intrusions: at A or at B.
If Blue intrudes at A, Red may defend the bridge at B (or play elsewhere). If Blue intrudes at B, Red may defend the bridge at A. Either way, Blue gains a bit of territory on one side of the bridge (but not the other). Blue is faced with the choice of which side of the bridge to intrude upon (if any).
Ziggurat intrusion
In the ziggurat, there are 8 potential intrusion points A–H:
Good intrusion
Which of several possible intrusions to play is a difficult question that depends on what else is going on on the board and what the intruding player would like to achieve. But generally speaking, it is usually better to intrude along the outside of a template, rather than in its center. A typical situation is the following:
Suppose Blue wishes to intrude into Red's ziggurat. If Blue intrudes at F (near the center of the template), Red reconnects at D. By further intruding into Red's bridge, Blue gets at most a 2nd row ladder escape along the right edge:
On the other hand, if Blue intrudes at D (near the edge of the template), Red reconnects at F, and Blue gets a 3rd row ladder escape. This is better for Blue.
However, not all intrusions along the template's outsides are good in all circumstances. For example, if Blue tries to intrude at A, Red may reconnect at D, forming edge template III2e and again leaving Blue with only a 2nd row escape:
If Blue intrudes at H, the situation is even worse for Blue: Red can respond as follows, capturing the entire corner and leaving Blue with nothing useful at all:
If Blue intrudes at C, Red can do the same thing, again leaving Blue with nothing:
Thus, Blue must carefully choose the best intrusion for the given situation.
Bad intrusion
Some intrusions are actually counterproductive. One example of this is intruding on the wrong side of a bolstered bridge, or more generally, of a bolstered template. For example, consider this situation.
If Blue intrudes at A, Red's response at B actually kills Blue's stone. Therefore, such an intrusion does not help Blue at all, but helps Red instead. See the article on bolstered templates for more details.
Invalid intrusion
When intruding into one of the opponent's templates or virtual connections, a player must be careful to take into account all of the possible ways in which the opponent is connected. For example, consider the following situation.
If Blue tries to intrude at A, Blue does not even threaten to disconnect Red, because Red is still connected by edge template III1b, threatening to play one of the two cells marked "*":
By definition, an intrusion must threaten to disconnect Red. Therefore, the move at A is not a valid intrusion at all. Instead, it is likely a wasted move.
One situation where an invalid intrusion can be beneficial is feinting.
Intrusion vs. blocking
An intrusion is not the same thing as blocking. By definition, an intrusion is into an area where the opponent is already virtually connected; therefore, the opponent can always re-establish the connection. By contrast, the goal of a blocking move is to disconnect the opponent. As mentioned above, intrusions are often best played on the outside of the connection, whereas blocking moves are best played near the inside. Thus, attempting a blocking move when the opponent already has a virtual connection is often catastrophic; conversely, attempting an intrusion when the opponent actually could have been blocked is also catastrophic. For this reason, it is very important to be able to read whether the opponent is already connected or not. The corresponding proverb is: "Don't defend the indefensible."
