Henry Wadsworth Longfellow

Once the Emperor Charles of Spain,
  With his swarthy, grave commanders,
I forget in what campaign,
Long besieged, in mud and rain,
  Some old frontier town of Flanders.

Up and down the dreary camp,
  In great boots of Spanish leather,
Striding with a measured tramp,
These Hidalgos, dull and damp,
  Cursed the Frenchmen, cursed the weather.

Thus as to and fro they went,
  Over upland and through hollow,
Giving their impatience vent,
Perched upon the Emperor's tent,
  In her nest, they spied a swallow.

Yes, it was a swallow's nest,
  Built of clay and hair of horses,
Mane, or tail, or dragoon's crest,
Found on hedge-rows east and west,
  After skirmish of the forces.

Then an old Hidalgo said,
  As he twirled his gray mustachio,
"Sure this swallow overhead
Thinks the Emperor's tent a shed,
  And the Emperor but a Macho!"

Hearing his imperial name
  Coupled with those words of malice,
Half in anger, half in shame,
Forth the great campaigner came
  Slowly from his canvas palace.

"Let no hand the bird molest,"
  Said he solemnly, "nor hurt her!"
Adding then, by way of jest,
"Golondrina is my guest,
  'Tis the wife of some deserter!"

Swift as bowstring speeds a shaft,
  Through the camp was spread the rumor,
And the soldiers, as they quaffed
Flemish beer at dinner, laughed
  At the Emperor's pleasant humor.

So unharmed and unafraid
  Sat the swallow still and brooded,
Till the constant cannonade
Through the walls a breach had made,
  And the siege was thus concluded.

Then the army, elsewhere bent,
  Struck its tents as if disbanding,
Only not the Emperor's tent,
For he ordered, ere he went,
  Very curtly, "Leave it standing!"

So it stood there all alone,
  Loosely flapping, torn and tattered,
Till the brood was fledged and flown,
Singing o'er those walls of stone
  Which the cannon-shot had shattered.