Aggression in duels
When fighting a single opponent, most people only consider direct offensive swings as "aggressive". While that is generally the case, one will also find that positioning and footwork can be aggressive. Moving towards an opponent's weakness is aggressive, even without swinging a sword. This will also be true in a line fight, but varied in scope.
What separates offense and aggression is that offense is applying pressure to the opponent with attacks, regardless of relative position--from a weak position, equal footing, or from a place of advantage. Aggression, however, is moving towards a place of advantage over the opponent (often with offense included). This forces an opponent to either fight from a disadvantage or react to get out of that disadvantage.
One thing to consider is actually the transition from a more passive or defensive posture into an aggressive attack. You might hear the term "explosive" used to describe fighters that sort of switch on to being aggressive quickly. By switching gears into a highly aggressive movement as quickly as possible, it makes it extremely difficult for an opponent to react. If, instead, one were to just start the fight by running directly at the opponent, they have time to prepare. However, by pacing one's self and waiting for the correct time to act, a quick burst of aggressive fighting might catch them flat-footed.
An example of this might be starting the fight defensively, while circling backwards and towards your sword side. This causes your opponent to constantly turn towards their shield side (assuming the same handedness). After a few moments of this circling, quickly switching directions and being aggressive will likely find their sword arm exposed as they are still trying to turn away from it. In this way, we see that aggression isn't always going to be a constant way to fight, but something that turns out to be essential at the right time.
With regard to footwork, the aggressive step is diagonally towards your opponent's sword side, as they have much less passive protection. Against two sword fighters, the more aggressive step is diagonally towards the opponent's lead leg side, for much the same reason--though it also serves to reduce their offense from their offhand weapon.
Aggression in line fighting
Much like a one on one, aggression in terms of line fighting is about moving towards the enemy's weakness. The difference is that the weakness here is often a gap in the enemy line, or another weak point, that might lead one into their backfield (which is the true weak spot). Also like our dueling considerations, aggression doesn't have to start at the beginning of the battle by running headlong into the enemy.
Many of the sort of shock trooper actions that I have discussed previously are best when executed at the correct time in the fight. For example, gaps don't often form in the enemy line until part(s) of the line are nearly engaged in the fight during the tail end of the maneuver phase of a battle. A shock trooper, looking to run a gap, has to wait and observe the enemy line and try to predict where and when that gap might be. Once the gap is formed, the shock trooper will assess the enemies nearby and try to time their attack to catch one of them unaware. This often involves noticing vision gaps at the area around the physical gap in the enemy line.
Other aggressive tactics, like line strafing (down the front of the line), don't require specific timing to start. When strafing a line, the aggressive fighter merely picks a target, usually based on vision gaps noticed, and makes a run for it between the two lines. The weak point being sought in this specific example is vision gaps, the first target simply being the first weak point noticed. As they move down the line, new vision gaps crop up and they can keep moving down seeking to deliver swings from each vision gap they find. If any of the vision gaps disappear (the shock trooper gets noticed), they simply block and keep moving down the line looking for the next vision gap.
Another case of aggression can be seen when looking at fighting while outnumbered. A single skilled fighter often moves towards the flanks of whatever group they are fighting against. Obviously, the flanks are a weak point of the line, and the single fighter gains advantage there. Therefore, moving towards the flanks here is aggressive and it forces the enemy to change their position (rotating to move their flanks).
Most people will generally consider aggression from only the standpoint of a sword and board fighter. However, aggression can also be used when using a support weapon. Not all of their aggression is direct from attacks, but from positioning their weapon along a line that will be able to exploit opening or force an opponent to cover them instead of their usual guard. Pointing a spear tip towards someone's sword side hip, for example, forces them to either lower their guard to block, back away, or risk the hit. Note that this aggression needs to be within the effective range of the weapon, and is amplified by attacking the target directly along this line. As I've covered many times when talking about glaive fighting, creating "presence" on the line is done by spreading attacks around to several targets. Ideally, engaging each of these targets in their weakest point (an exposed leg, sword side hip, or shield side if it is low) will further amplify the effect.
Confidence, Threat, and Perception
Other aggressive tactics, like line strafing (down the front of the line), don't require specific timing to start. When strafing a line, the aggressive fighter merely picks a target, usually based on vision gaps noticed, and makes a run for it between the two lines. The weak point being sought in this specific example is vision gaps, the first target simply being the first weak point noticed. As they move down the line, new vision gaps crop up and they can keep moving down seeking to deliver swings from each vision gap they find. If any of the vision gaps disappear (the shock trooper gets noticed), they simply block and keep moving down the line looking for the next vision gap.
Another case of aggression can be seen when looking at fighting while outnumbered. A single skilled fighter often moves towards the flanks of whatever group they are fighting against. Obviously, the flanks are a weak point of the line, and the single fighter gains advantage there. Therefore, moving towards the flanks here is aggressive and it forces the enemy to change their position (rotating to move their flanks).
Most people will generally consider aggression from only the standpoint of a sword and board fighter. However, aggression can also be used when using a support weapon. Not all of their aggression is direct from attacks, but from positioning their weapon along a line that will be able to exploit opening or force an opponent to cover them instead of their usual guard. Pointing a spear tip towards someone's sword side hip, for example, forces them to either lower their guard to block, back away, or risk the hit. Note that this aggression needs to be within the effective range of the weapon, and is amplified by attacking the target directly along this line. As I've covered many times when talking about glaive fighting, creating "presence" on the line is done by spreading attacks around to several targets. Ideally, engaging each of these targets in their weakest point (an exposed leg, sword side hip, or shield side if it is low) will further amplify the effect.
Confidence, Threat, and Perception
One thing that I believe several fighters have issue with when trying to be aggressive is actually confidence in what they are doing. Being aggressive while presenting a more timid appearance and/or hesitating doesn't have the same effect as a determined appearance. It may be that one is unsure if their plan will be successful, and this impacts how they appear to the enemy. To alleviate this problem, one has to commit to their action fully, without worrying too much about the outcome.
Threat is roughly how the enemy perceives one's effective range. This effective range is amplified by things like longer weapons or previous encounters of one running, rushing, or otherwise being aggressive. Someone that never does more than walk forward and never runs gaps will have a much lower perceived threat than someone that is constantly strafing lines and breaking gaps. This threat range increases the effects of aggressive maneuvers.
For example, several folks out there recognize my shenanigans when strafing lines or pushing gaps. This causes people to call out when I am approaching gaps or maneuvering down a line much faster and more often than when someone doesn't engage in the fight that way on a normal basis. While my personal success ends up being lower due to the call, my actual impact on the team via aggression is amplified. I force their line to move against me, or deal with me running around in their backfield. (Though, I do sometimes miss the days when no one realized what I was about to do...)
For example, several folks out there recognize my shenanigans when strafing lines or pushing gaps. This causes people to call out when I am approaching gaps or maneuvering down a line much faster and more often than when someone doesn't engage in the fight that way on a normal basis. While my personal success ends up being lower due to the call, my actual impact on the team via aggression is amplified. I force their line to move against me, or deal with me running around in their backfield. (Though, I do sometimes miss the days when no one realized what I was about to do...)
Misc
Much of being an aggressive fighter is actually just being an observant fighter that is willing to take some risks.
Observing a weakness in the enemy doesn't mean much if you don't "just go for it".
Observing a weakness in the enemy doesn't mean much if you don't "just go for it".
Aggression is one of the tools used to manipulate the enemy into the position of your choice. That includes a line of enemies.
Aggressiveness does not have to be recklessness.