but a curse if you disobey the commandments of the LORD your God and turn aside from the path I command you today by following other gods, which you have not known.
The curse comes if they disobey, turn from the commanded path, and follow other gods they have not known.
It defines the negative side of the choice in terms of disloyalty and idolatry.
26See, today I am setting before you a blessing and a curse — 27a blessing if you obey the commandments of the LORD your God that I am giving you today, 28but a curse if you disobey the commandments of the LORD your God and turn aside from the path I command you today by following other gods, which you have not known. 29When the LORD your God brings you into the land you are entering to possess, you are to proclaim the blessing on Mount Gerizim and the curse on Mount Ebal. 30Are not these mountains across the Jordan, west of the road toward the sunset, in the land of the Canaanites who live in the Arabah opposite Gilgal near the Oak of Moreh? 31For you are about to cross the Jordan to enter and possess the land that the LORD your God is giving you. When you take possession of it and settle in it, 32be careful to follow all the statutes and ordinances that I am setting before you today.