To understand Abraham Lincoln's strategy we first must understand that the Civil War was started over secession from the Union, not slavery. (Slavery was legal throughout the united States, but it was the Southern States that was so economically dependent upon slavery as a labor force.) Legally, the war was about rebellion from authority.

During the first 2 years, in the Northern States there was not a lot of support for the war, and the South was winning battle after battle.

Property rights for slave owners were law and legal, but as spoils of the war, contraband could be kept. Lincoln allowed run away salves to stay in interment with the Union Army as contraband, captured enemy property. From this issue came the next strategic move by Lincoln.

In the Fall of 1862 President Lincoln wrote a preliminary "Emancipation Proclamation." It said that in States where there was rebellion against the Union (if not repatriated by January 1, 1863), the slaves that supported the war effort would be declared "henceforth and forever free." This was a total change in strategy for the war. Note that all slaves were not set free until the 13th Amendment was passed by Congress later.

The Emancipation Proclamation gave slaves that were serving the enemy of the Union the "green light" to run away and become free men in the Union States, and so they did. The Underground Railroad, as it was called, helped families to escape slavery. Perhaps there were a million slaves (of the known 4 million) who chose to run to freedom.

Lincoln's bold act made an issue of slavery and won support for the war, while weakening the enemy's ability to sustain the fight. Abraham Lincoln in his wisdom did advocate breaking the law, but used the law to break the enemy.

Abraham Lincoln

Attorney at Law
President of the United States
Commander of Union Army
Emancipator of Slaves
National Statesman
Christian Hero

So, the Emancipation Proclamation broke the power of enslavement in the enemy's camp. Lincoln did not coerce slaves to defy the law. He did not circumvent the Constitution by abolishing slavery, rather Lincoln through military powers disallowed in the rebellious States the right to have slaves.