John Gill Commentary Deuteronomy 30:18

"I denounce unto you this day, that ye shall surely perish; ye shall not prolong your days in the land, whither thou passest over the Jordan to go in to possess it." — Deuteronomy 30:18 (ASV)

I denounce unto you this day that you shall surely perish
&c.] By one judgment or another; this he most solemnly averred, and it might be depended upon that it would certainly be their case; see (Deuteronomy 4:26);

[and that] you shall not prolong [your] days upon the land, to which
you pass over Jordan to go to possess it ;
but be cut short by death, of one kind or another, by sword, or famine, or pestilence, or be carried into captivity; one or other of which were frequently their case.