Grenades

In fact both of these weapons were to be used chiefly to defend the grenadiers: those men tasked with the bombing of trenches and positions using grenades of various types.  Bombing parties grew in number and frequency as the war progressed and formed a major component of any infantry attack by the war's close (although US forces used them less, chiefly on account of supply shortages).

The British bombing team usually consisted of nine men at a time: an NCO (non-commissioned officer), two throwers, two grenade-carriers, two bayonet-men to defend the team and two 'spare' men for use when casualties were incurred.

As an attack or raid reached an enemy trench the grenadiers would be responsible for racing down the trench and throwing grenades into each dugout they passed: this invariably succeeded in purging dugouts of their human occupants in an attempt at surrender (often not accepted as they were promptly shot or stabbed).  

However, this only occurs if the bombing party was able to survive the thick defensive shield of riflemen and machine guns.  Often times, the attack group never makes it close enough to the enemy trench to get in range of throwing grenades.  The attackers were usually cut down by the longer-ranged machine guns and rifles.

When Britain entered the war on 4 August it did so with just one type of grenade in its armory (suitably named 'Mark 1'), and not very many of those.  As with the machine gun the British high command could not see much use for the hand grenade.  This situation soon changed however; indeed, within a year Britain was producing up to half a million hand grenades each week (with an average of 250,000).