Final answer:
Abraham Lincoln was referring to the United States being divided over the issue of slavery when he said 'a house divided against itself cannot stand.' The house he spoke of is a metaphor for the country, symbolizing the divide between the northern and southern states.
Step-by-step explanation:
When Abraham Lincoln declared that "a house divided against itself cannot stand," he was referring to the nation being divided over the issue of slavery. The house he spoke of is a metaphor for the United States of America, symbolizing the division between the northern states that were against slavery and the southern states that were in favor of it. Lincoln believed that the country could not continue to exist if it remained divided on the issue of slavery and that it would either become entirely free or entirely slave.