Rashi, from Bereishis Rabah 80:3: This hints to Chamor telling Yakov to give his daughter to Shechem. Malbim - this parable was common among Bnei Yisrael, to compare Shechem to a thorn tree. Just like he caused harm to himself and his entire nation, so you cause harm to Yosef and to Bnei Yehudah!

Radak: Even if you wanted to join to me peacefully, it would be disgraceful for me, and all the more so via war!

Rashi, from Bereishis Rabah 80:3: This hints to Bnei Yakov [killing the city and] stripping the dead bodies. It was a parable of disgrace, to compare Amatzyah to a thorn tree and (Yo'ash) himself to a cedar. Also you, because you became haughty, my troops will trample you!