To attack during the offensive phase, offensives must purchased during the previous seasonal turn for the next 3 months of game time. If you run out of offensives between seasonal turns, offensives can be purchased, but these impromptu offensives are very expensive.
Once the offensive has been paid for a group of units needs to be activated and then they may move and attack. The number of hexes this group may attack is determined by how they were activated. One hex maybe attacked if activated by a stack or general and up to three hexes by HQ. Each major country has different HQ units depending on their historic command and control structure. USSR begins with horrendous HQ units that can be swapped out as USSR improves its C&C over time and Germany has several large radii HQs with lots of hex attack potential.
When a large number of hexes need to be attacked at one time, multiple HQs can be activated for the cost of 2 offensives and 2 economic points. This is called a Combined Offensive. Combined Offensives can attack a number of hexes equal to the attack rating of all HQs activated.
In ETO there is no AXIS attack turn, then ALLIES attack turn, like most W.W.II games. In ETO the game flows back and forth between offensives because both sides SHARE the same offensive phase. For example, the country that purchased (during each seasonal turn, 4 per year) the most offensives for next three months is given the initiative (the ability to launch offensives first) and after one side has attack(s) the other side then gets an offensive or combined offensive. This creates a realistic ebb and flow feel to ground combat. Offensives occur on both sides along with counter offensives until either side runs out of offensives or units to activate. Counter offensives can occur in the middle of large-scale enemy offensives to spoil or thwart them altogether. All ground units may only be activated once during a monthly turn, so after your opponent has shown his hand, if you have units left to activate, you can take advantage of this. Air units that have limited abilities once they have been used to support an offensive, so each side must husband their aircraft accordingly.
Air Combat
For example, a large group of Hurricanes (4 strength and 2 quality) is attacking a small group of ME109s (2 strength and 3 quality). The attack rating of the Hurricanes is 4 (strength) +2 (quality) = 6 minus 3 (for the ME109s quality rating) which is modified to a 3. Results are then rolled on a die: 3 to abort, 2 to abort/hit, 1 to abort/hit. The ME109s attack rating is 2 (strength) + 3 (quality) = 5 minus 2 (for the Hurricanes quality rating) which is modified to a 3. Results are then rolled on a die: 3 to abort, 2 to abort/hit, 1 to abort/hit. A full strength group of ME109s would usually abort/hit a group of Hurricanes with an attack rating of 4 (strength) + 3 (quality) = 7 minus 2 (for the Hurricanes quality rating). Results are then rolled on a die: 5 to abort, 4 to abort/hit, 3 to abort/hit, 2 to abort/hit, 1 to abort/hit.
With home field modifiers, air battles over airbases are very interesting (for example the Battle of Britain). Having your aircraft shot down over home territory is also preferred because Pilot recovery is allowed which is a less expensive way of rebuilding aircraft during the same monthly turn it occurs. Normally you have to wait until seasonal turns to repair/build new units. For example, this becomes important when America begins its strategic campaign with numerous B-17s pounding Germany or the UK defending its homeland against Germany air attacks.
Combat Results Table
Armor Effects
Breakthroughs allow armor stacks to attack first and then move (they are allowed a breakthrough movement of 1 hex or by using the rating of the accompanying General which can be up to 5 hexes), however the armor can not move during the combat phase (along with infantry). Breakthroughs allow armor to clear lanes, which frees infantry to move instead of being blocked by ZOC. Armor that performs a breakthrough can attack during the combat phase (along with infantry).
Exploitation occurs differently. No movement is allowed during the breakthrough phase, however, armor can move normally during the combat phase (along with infantry). After a successful attack, which included armor stacked with a General, the armor can move and attack again by itself or with other exploiting armor. The distance armor can move during Exploitation is determined by the Exploitation rating of the general that is stacked with the armor. For example, both Patton and Guderian have a 5 hex rating.