Both the British Army and Royal Navy issued a daily rum ration during WWI. In the Royal Navy, the ration depended on rank. The typical seaman would receive an eighth of a pint of rum (or another spirit if rum was unavailable) each day. This was mixed with water, in a ratio of 3 parts water to one part rum. Petty officers would receive a full quarter-pint of rum, with no dilution. The ration rum was much stronger than most modern rums, at 95% proof. All sailors above the age of twenty were eligible to receive the ration. However, should they choose not to receive it, they would receive a small extra payment - 'grog money' - in lieu. The rum ration was given out, with no small degree of ceremony, between 11am-12 noon each day. Extra rations could be given out in extreme circumstances, at the discretion of the captain. Extra rations could also be given out at the command of a senior officer to celebrate events like a royal inspection or a victory in battle - this used the signal 'Splice the Mainbrace'.
The Army, meanwhile, issued a smaller ration. The Army's ration was a quarter-gill, or one-sixteenth of a pint. Unlike the Navy, this was not part of the standard ration. It was, instead, supposed to be a compensation for arduous conditions; the issuing of the ration to a division had to be approved by the division's commander based on advice by his medical officers. In practice, the vast majority divisional commanders did approve the ration. Only a few did not, and they were generally hated by their men. The rum ration was typically given in the morning, at 'Stand-To'. It was also frequently given before battles, as a way to bolster courage. It could also be issued, at the discretion of commanding officers, following difficult conditions, like heavy shelling or sentry duty in bad weather. The rum was issued neat, from earthenware jars, though it was frequently drunk in tea.
Also interested. Are you asking specifically about WW1 and the involved nations, only during those years? Or Any time/anywhere else in history?