Right at the start of the speech, MLK makes an allusion, or reference to history, recalling the Emancipation Proclamation (2.1). In historical texts as well as literature, specific references to other texts or events can help place the document in a larger context.

Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation. (2.1)

The Emancipation Proclamation officially freed all of America's slaves. It was the most important moment in American history since the Revolution. So this allusion places "I Have a Dream" in some upper-tier company. The audience definitely felt the weight of the moment, like they were witnessing history.

This allusion also fits in to MLK's idea of incremental historical progress. First, slavery ends; then, we deal with discrimination; eventually, race and racism won't matter at all. Starting the speech with an allusion to the end of slavery also emphasizes the point that there are continuing obstacles facing African Americans besides slavery.

For those who were saying, "Hey, there are no more slaves, racism is over," MLK's use of an allusion was a wake-up call.