The subject, taken from early Greek history, was especially popular in the renaissance and baroque period.  According to the legend the Amazons were war-like women who constantly invaded ancient Greece.  Their military prowess was associated with bizarre customs which involved meeting with men of another race at certain seasons and killing or maiming any male offspring.  The surviving females then had their right breast removed as it was thought it interfered with their fighting prowess.  The Amazons were entirely mythical and many legends about them grew up in 5th century BC Greece.  They were finally defeated by Theseus and most of the paintings of the subject show the violent action where the women are overcome.  The painting itself is now seen to be one of Rubens very earliest works, whilst still under the influence of his master, Otto van Veen