Final answer:
Abraham Lincoln issued the Emancipation Proclamation to preserve the Union by undermining the South's ability to wage war.
Step-by-step explanation:
One of the causes for Abraham Lincoln issuing the Emancipation Proclamation was the preservation of the Union. By freeing the slaves in the rebelling states, Lincoln hoped to undermine their ability to wage war and weaken the South. This was a strategic decision made to cripple the South's ability to fight against the Union and secure victory.