Congress is supposed to make laws and treaties, the president is supposed to carry them out. Can he secure the border by himself? Theoretically no if he is executing laws as written by Congress. The problem is that Congress has made such a mess of immigration laws, and everything else, it's impossible to say what they mean anymore, and this is intentional. They pass muddled laws that can be interpreted any way the current regime feels inclined. On top of that are the career bureau managers who write regulations and protocols out their ass as they deem cool, secure in their fiefdoms. Then you have the scotus who have absolutely gone batshit crazy for some reason, Biden can have an impact on immigration, good or bad but the system is driven more by moneyed interests and power centers than any one person. Can Biden start a war? Congress is charged with declaring war, how is that playing out here? Instead of a table with three legs ( checks and balances) start thinking of 1000 "public figures" in a huge circle with each of them pointing to the one on his left.