What is fight over Trump's border wall about?

President Donald Trump has partially shut down the US government and is threatening to declare a national emergency as he pressures Congress for money to build a wall on the US-Mexican border to stem a surge in illegal immigrants.Trump, who is to make a national address Tuesday night on the "crisis," says the country is being flooded with drugs and violent gangs.Trump's opponents say he is exaggerating the issue and that a massive wall is inefficient and not worth closing the government for.