thorns

Know for a certainty that the LORD your God will no more drive out any of these nations from before you; but they shall be snares and traps unto you, and scourges in your sides, and thorns in your eyes, until ye perish from off this good land which the LORD your God hath given you - Joshua 23:13

But if ye will not drive out the inhabitants of the land from before you; then it shall come to pass, that those which ye let remain of them shall be pricks in your eyes, and thorns in your sides, and shall vex you in the land wherein ye dwell - Numbers 33:55

Wherefore I also said, I will not drive them out from before you; but they shall be as thorns in your sides, and their gods shall be a snare unto you. - Judges 2:3

Disobedience to God yields harvest of thorns

Now these are the nations which the LORD left, to prove Israel by them, even as many of Israel as had not known all the wars of Canaan;  Judges 3:1

not all the Canaanites were driven out of the land, some were left by the Lord to "prove" the following generations who inherited the land and had not fought for it.  we are quick to defend something hard won, but just as quick to take for granted and loose something easily gained...

"And there will be no more for the house of Israel a prickling brier or a painful thorn from any round about them who scorned them; then they will know that I am the Lord GOD." Ez 28:24

other nations buffet Israel as a thorn in the side 

And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.  2 Cor 12:7

the Lord may not remove all of the messengers of the enemy but leave some to "prove" us, keep us humble, and keep us in good spiritual shape and fighting readiness by giving us something for sparring and preparation for coming battles...

Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? - Matthew 7:16

And that which fell among thorns are they, which, when they have heard, go forth, and are choked with cares and riches and pleasures of this life, and bring no fruit to perfection. - Luke 8:14