Just one month after writing this letter, Lincoln issued his preliminary Emancipation Proclamation, which announced that at the beginning of 1863, he would use his war powers to free all slaves in states still in rebellion as they came under Union control. Lincoln scholar Harold Holzer wrote in this context about Lincoln's letter: "Unknown to Greeley, Lincoln composed this after he had already drafted a preliminary Emancipation Proclamation, which he had determined to issue after the next Union military victory. Therefore, this letter, was in truth, an attempt to position the impending announcement in terms of saving the Union, not freeing slaves as a humanitarian gesture. It was one of Lincoln's most skillful public relations efforts, even if it has cast longstanding doubt on his sincerity as a liberator." Historian Richard Striner argues that "for years" Lincoln's letter has been misread as "Lincoln only wanted to save the Union." However, within the context of Lincoln's entire career and pronouncements on slavery this interpretation is wrong, according to Striner. Rather, Lincoln was softening the strong Northern white supremacist opposition to his imminent emancipation by tying it to the cause of the Union. This opposition would fight for the Union but not to end slavery, so Lincoln gave them the means and motivation to do both, at the same time. In his 2014 book, "Lincoln's Gamble", journalist and historian Todd Brewster asserted that Lincoln's desire to reassert the saving of the Union as his sole war goal was in fact crucial to his claim of legal authority for emancipation. Since slavery was protected by the Constitution, the only way that he could free the slaves was as a tactic of war—not as the mission itself. But that carried the risk that when the war ended, so would the justification for freeing the slaves. Late in 1862, Lincoln asked his Attorney General, Edward Bates, for an opinion as to whether slaves freed through a war-related proclamation of emancipation could be re-enslaved once the war was over. Bates had to work through the language of the Dred Scott decision to arrive at an answer, but he finally concluded that they could indeed remain free. Still, a complete end to slavery would require a constitutional amendment.