At the dawn of creation, elves and eladrin were a single race dwelling both in the Feywild and in the world, and passing freely between the two. When the drow rebelled against their kin, under the leadership of the god Lolth, the resulting battles tore the fey kingdoms asunder. Ties between the peoples of the Feywild and the world grew tenuous, and eventually the elves and eladrin grew into two distinct races. (Player's Handbook, pg. 41)
Unlike the dwarves, elves are a race that were a consequence of war.
While the eladrin remained in their crystalline towers, the elves entered into the world to live amongst its trees in the forest. As the races grew distinctively apart, the elves slowly learned to live in harmony with their new homeland. Although the spirit of the fey still lives in them, their being grows closer to the natural world.
Elves revere the natural world. Their connection to their surroundings enables them to perceive much. They never cut living trees, and when they create permanent communities, they do so by carefully growing or weaving arbors, tree houses, and catwalks from living branches. (Player's Handbook, pg. 41)
Unlike their eladrin cousins, elves eventually adapted closer to the primal spirits and call forth its powers rather than relying on the arcane. Instead of living in beautifully architecture and fortified homes, elven communities are mostly defenseless unless they are within natural barriers.
Their bond with nature had healed their wounds from the war with the drow and once again the elves had found peace and harmony.
Since then, elves have become a simple people, enjoying the simple life provided by the bounties of nature and consider themselves as guardians of their natural homes. They may be care-free but are not complacent. Their eyes watch closely for any threat that would dare come near their borders.
Elves are very cautious and suspicious towards armed strangers in their forest. They are curious at best but most of the time, they would shadow the alien visitors for days without them knowing to gauge their intentions. They will pay close attention to details like what are they wearing, the weapons they carry, their actions and most importantly, how they treat the nature surrounding them. Anyone that makes it clear that they will bring destruction to the forest will confirm the elves' suspicions and they will report back to their community that a threat has appeared.
The elders of the community would discuss among themselves on how this threat is perceived and what punishments would be suitable for these defilers. They will no tolerate anyone that put the survival of the community at stake.
Elves in Times of War
Despite how unpleasant war can be, a threat to their homes, families, or friends can make elves grimly serious and prompt them to take up arms. (Player's Handbook, pg. 41)
All elven communities rely on nature to survive, whether it's by foraging or hunting and all adult elves are either proficient in using a weapon or have good knowledge on the lay of the land as part of their survival skills. Some elves even form good relationships with the beasts and the plants of the forest and could call upon them as allies in times of need. This makes every elf have something to contribute in the time of war, physically or tactically.
Slender and agile, elves revere nature and roam the wilderness, hunting creatures that threaten their lands. (Monster Manual, pg. 106)
Elves generally don't have a word for 'war' which is an ancient concept that predates their race before they split from the eladrin. Instead, when elves go to war, they use the word 'hunt' to describe such a conflict. The mission of the 'hunt' is to kill their enemy (which they call them as 'prey') before they find their community. Elves rarely evacuate their homes since it can be difficult to find another suitable place in the forest that can sustain them.
There is another concern when an elven community is on a 'hunt'. Elves are very prone of wandering into the wilderness for weeks and months before returning to their community because of wanderlust. It is often when the elders call for a 'hunt' that many of their own are somewhere in the forest. This is where those elves that have ties with the natural world would send our messengers to track down their kin through animals and plants. Wandering elves would eventually heed the call and rush back to the defense of their homes.
If there is a 'prey' that is great enough to threaten another community, elders would send out runners and arrange a meeting with elders from that community for help. Although each community is still responsible for their own defenses but cooperation is very common among elvish tribes to bring down a greater 'prey'.
Elves don't have a formal way of organizing an army since their numbers are often small and they don't have a strict chain of command. They are simply organized into small groups consisting of archers and scouts that are called 'packs'. Most elven hunters are very deadly archers but the scouts are melee combatants that are known for their two-bladed fighting skills. Those that are bonded to nature can form their own packs with their animal companions that are equally aggressive and ferocious in bringing down their prey.
There is no hierarchy within a pack and they operate based on respect and mutual understanding. Each pack must report to a 'Packmaster' who coordinates their attacks, discusses the tactics and places where they would waylay on their prey and plan a line of retreat. Although the packmaster is the one with the most experience and knowledge, they do not override any suggestions made by a younger member from a pack if they have a better plan. These decisions are made based on very strong agreements and understanding among the packs that will executed them.
Elves in the Battlefield
Elvish packs rely heavily on stealth and camouflage. When they arrive on a pre-determined spot where a prey is bound to pass through or set up camp, the elves would quickly set up an ambush. They find as many positions with a lot of cover as they can and each pack takes their place in each of them. Once they are in position, the packs would wait silently only communicating by imitating animal noises that blends with the environment or using animals for those who are naturally inclined.
Elves can be very patient when waiting for an enemy to approach and are not unnerved even if facing against a number that is much greater than their own. They are very confident of their strengths and they know that nature is always on their side. Elves strike when the prey is most unprepared or not on their guard even if the prey has been there for hours.
Elvish attacks are sudden and end quickly. Archers from unknown corners would start firing their arrows with deadly accuracy, killing their prey from every angle on the battlefield and scouts would charge out to slaughter the enemy. Once they have lost the element of surprise, the packs break for the nearest cover and leave behind a trace of their fallen foes and not a single traceable track.
After that, the packs would make their way to rendezvous at an agreed location and plan for their next attack until the prey is taken out or driven from their homes shrieking in fear of elvish ambush.
Tactics & Strategy
Wild and free, elves guard their forested lands using stealth and deadly arrows from the trees. They build their homes in close harmony with the forest, so perfectly joined that travelers often fail to notice that they have entered an elven community until it is too late. (Player's Handbook, pg. 40)
The basis of an elvish hunt is guerrilla warfare. The elves would either use fear to drive away their prey from their homeland or constantly pepper them arrows until the prey is killed. Along the way, elves are also master hunters and lay booby traps for areas that are too small for an elven ambush.
The main objective of the 'hunt' is to prevent it from finding or reaching their community, so elves would also place false tracks that would lead their prey on a detour or to a perfect spot where another pack is waiting for an ambush.
Packs are also allowed to maneuver independently around the prey and make small scale skirmishes with them but never stray too far from the rest of the other packs and the pack master. Packs also don't generally stay in one place for long in fear that the enemy might send out vanguards to hunt down the packs.
Packs also take turns to attack the enemy as they have a rotating system which keep the pressure on the prey. Some packs and their packmaster follows the prey closely to make contact while another group of packs and its packmaster could be preparing an ambush in another site. The lack of a need to sleep for elves also means that their skirmishes can happen at any time of the day and night and can seem relentless to the enemy.
If the elves are not able to drive the prey as they reach closer to the community, all the packs would gather together for one last final stand.
Ages ago, after they entered the world and became separated from the eladrin, elves found it necessary to survive in the hostile wilds. Grace and awareness became their strengths. The bow became their chosen weapon for hunting and stealthy warfare. (Martial Power, pg. 68)
Elven archers are legendary for their accuracy and deadliness. It is said that entire armies can easily be mowed down by elven arrows alone if they present themselves as such an opportunity. Elven archers are even known to hit hard on the run, even while retreating, enemies are hesitant to pursue knowing that an arrow could lodge in between their eyes as a reward for trying.
Legendary elvish archers are said to be so spiritually linked with their arrows and bow that they could guide them to seek an enemy as if it has a mind of its own. Other legends speak of an elvish arrow being able to take down multiple foes at once.
Elven battle armor (Player's Handbook, pg. 230) tends to be light and are usually made of leather or hide which are worn by elven archers so as not to impede on their movement for stealth or during a quick retreat. Packmasters wear Wildleaf Armor (Adventurers' Vault, pg. 55) which is fashioned from fallen leaves that gives them greater maneuverability over even the most treacherous terrain.
Scouts wear heavier armor who needs them for greater protection in melee but it is normally not any heavier than chainmail. However, it is known that elves who live near by the river and seas wear heavy Coral Armor (Adventurers' Vault, pg. 43) which allows them to swim and breath in water. Elves are one of the few armies that can employ amphibious tactics because of this.
For those that are naturally inclined to the forest, they favor the Loamweave Armor (Adventurers' Vault, pg. 48) which is cloth weaved in together with soil that grants them greater control over the earth.
Every elf also wear a pair of enchanted elven boots (Player's Handbook, pg 246) and an elven cloak (Player's Handbook, pg. 250) which makes them stealthy or hide better under cover.
Elven beast masters also bring packs of grey wolves (Monster Manual, pg. 264) for a 'hunt' and sometimes they form their own packs.