Isaiah 52:12 says

But you will not leave in haste or
             go in flight;
for the Lord will
                 go before you,
    the God of Israel will
                 be your rear guard.

Comment/Devo:
~ Isaiah is recognized by many scholars as a masterful writer. His Hebrew, while among the most difficult in the Bible, is also among the most majestic and glorious. J. Sidlow Baxter has said of Isaiah – “What Beethoven is in the realm of music, what Shakespeare is in the realm of literature, what Spurgeon was among the Victorian preachers, that is Isaiah among the prophets.”

~ I ended the excerpt above with verse 12. That is the verse that is of interest to me as it contains an imbalance. It balances the word “before” with the word “rear-guard”.
– It does not balance “before” with “after” as sometimes you hear or read.
– It does not balance “go ahead” with “behind” as sometimes you hear or read.
– It does not balance “vanguard” with “rear-guard” as some write.

No. Isaiah is a writer par excellence, who very carefully chose his words and deliberately introduced an imbalance.

Cutting to the Chase: Here is what I see as going on.
1) The Idea: When Israel walked around the wilderness for 40 years, who was at the back?
 ~ At the back were the elderly, the disabled, the sick, the weak, the tired, the disabled, those who were unclean (ceremonially and otherwise), the lepers and so on. Those who were somehow disadvantaged were at the back.

These folks needed special protection and could easily fall behind.

This is where the rare word, rearguard, comes in.

The word, rearguard encompasses two idea. One is that of protection and another is that of gathering.

~ Per protection, it could be needed as those at the back were particularly weak and vulnerable.

In fact, in Deuteronomy 25:17-18, we are told that when the Amalekites attacked Israel, they deliberately attacked the rear ranks, because this is where the weakest and most tired were.

~ Per gathering, it meant that when there were those who were struggling to keep up, or when they fell behind, then the rearguard would gather them up.

Meaning, God as Rearguard protected and gathered the last of the last, the weakest of the weak, the most vulnerable of His people. He was with them in a special way.

~ This has application today. When you think of the shut-in or the believer who is on a hospital bed on Christmas night, and not at some festivity, or when you think of another believer whose tight finances, keeps him or her from engaging in various church activities, know that God is with them in a special way. His work begins there before it begins at the Christmas tree.