If I should die, think only this of me:
That there's some corner of a foreign field
That is for ever England.  There shall be
In that rich earth a richer dust concealed;
Brooke opens his poem with the conditional command ‘If I should die … ‘ He clearly accepts the possibility of his own death.  It establishes immediately a tone of bravery and assertiveness yet resignation that continues throughout the poem. 
 This line seems particularly poignant because Brooke did indeed die, not far into the war; although, it was of an  infected mosquito bite , and occurred before he’d actually entered combat. 
 The qualification ‘only’  establishes the patriotism that motivated Brooke, as well as other young soldiers at the early, idealistic stage of the war.  He wants to be seen as devoted to his country and not mourned. As the poem will go on to explain, he values his country’s blessings.
The Soldier