20 Like the nations the LORD destroyed before you, so you will be destroyed for not obeying the LORD your God.

Read Deuteronomy 8:20 Using Other Translations

As the nations which the LORD destroyeth before your face, so shall ye perish; because ye would not be obedient unto the voice of the LORD your God.
Like the nations that the LORD makes to perish before you, so shall you perish, because you would not obey the voice of the LORD your God.
Just as the LORD has destroyed other nations in your path, you also will be destroyed if you refuse to obey the LORD your God.