Originally appeared here and referenced at The Tactical Hermit. -NCS
Assaulting a fortified position is a high risk operation with a potential for extreme causalities.  The attacking side not only has to contest with the defenders advantage of pre-placed defenses, pre-sighted artillery, pre-set plans and pre-stocked war supply but also overcome any local superiority in firepower and manpower.   The attacker has to rely on one of two sets of thought in assaulting, either baring even more powerful and more superior forces into assaulting the fortified position (with a cost-benefit ratio favoring the defender) or to utilize the only remaining options left to an attacker for an economic attack.  Surprise, speed, boldness and solid intelligence.
Considering the defender is in a fixed position, intelligence gathering on the defenders capabilities is a process of obtaining the right signals, through signal interception or electro-optical observation (thermal, night vision, optics, etc.).  Human intelligence sources (such as locals, insiders, etc.) and other means can be utilized to properly predict the type of assets and potential reactions a defender may have at their disposal.  This can help create contingency plans during the assault.
The more intricate the attack plan is, the more likely it’ll fail.  The enemy, however, may not allow simple maneuvers  to overtake him and therefore an detailed and highly orchastrated attack plan may be in order to tip the enemy over.
Assaulting a Fixed Position:
Assaulting a fixed position (sandbag, small bunkers and trench emplacements) requires speed and surprise.  Initial deployment of heavy fire-support elements (grenade launchers, mortars, rocket launchers, heavy machine guns, etc.) will heavily influence the shock and surprise of the opening attack.  The first strike must target defending enemy heavy weapons emplacements and defensive obstructions in an effort to destroy or suppress these positions to allow assault infantry to commence with the next phase of the assault.

The modernized version of the Brecourt Manor assault.  Not to scale.

The opening volley of mortars, rocket launchers, etc. against enemy defensive positions should render most critical defense weapons obsolete if not suppressed enough to allow the main assault to succeed with minimal/acceptable losses.  The main assault is initiated during the second phase, continually supported by the pre-placed heavy weapons.  Attacking infantry will assault on awkward yet simple vectors; particularly in any potential enemy blind spots or flanks (such as rolling up on flanks, fighting down an enemy trench, rapid  deployment in enemy rear guard, etc.).
The success of the operation relies on speed.  The element of surprise is a fleeting window, initial success has to be drastic and monopolized upon to completely dismay the enemy before they can muster reinforcements for a counter-attack or reorganize defensive lines and formations.
Assaulting a Fortified Position:
Assaulting a bunker or a reinforced building requires the former school of thought in assaulting; relying on the deployment of superior weapons and forces to that of the defender in terms of numbers, firepower, and capability.  This does not necessarily always mean more manpower advantage, but it definitely means heavier weapons such as flame throwers, grenade launchers, high explosives, fuel air bombs, and other modern siege weaponry.
The defender has an extraordinary defensive position against small unit tactics considering the layers of defense a fortified position, such as a bunker or a building offers.  Obfuscation of movement and a terrain modifier that severely limits any numerical advantage an attacker may have.  In such situations its usually better to take out the bunker or garrisoned structure completely through heavy artillery or air strikes, but if an attacker must commit troops to retake a bunker or structure the attack relies on fast and hard hitting assaults utilizing specialized siege weapons.  Flame throwers, grenade launchers, stun weapons (such as flash bangs), smoke grenades, etc.

Although speed and surprise still play a crucial role in urban breaching and siege operations, the real game changer is the anti-garrison heavy weapons deployed (such as flame throwers, rockets or grenades).

For example, a reinforced building in an urban setting can be assaulted in a multitude of ways.  One common way is by deploying a short-range high-explosive rocket (such as a TOW missile) to initially be fired into the building (or several volleys at least) before a rapid assault team
The invention of modern weaponry has made fortifying a fixed position unpleasant in any wartime situation.  Not only because the maneuver warfare can cut off all lines of supply and communication but because the magnitude of destruction modern weapons can dispense has made holding any single fixed position too risky and costly.