Final answer:
The Emancipation Proclamation was criticized for focusing more on protecting the Union than freeing slaves, as it only applied to areas still in rebellion against the Union and not all slaves. Critics argued that it had limited impact on ending slavery.
Step-by-step explanation:
The aspect of the Emancipation Proclamation that caused some to believe it was more focused on protecting the Union rather than freeing slaves is that it only freed slaves in areas still in rebellion against the Union, and not all slaves in all areas. Thus, In essence, the Proclamation liberated slaves where the Union had no enforcement power but left slavery untouched in areas where the Union had control.
This meant that slaves in states under Union control, including the North, were not affected by the Proclamation. As a result, critics argued that the Proclamation had limited impact on ending slavery and was primarily aimed at undermining the Confederacy's ability to wage war.