frame创建新变量

I have a dataframe with around 1.5 million rows and 5 cols. One variable (VARIABLE) is of this type NATIONALITY_YEAR (e.g. SPAIN_1998) and I want to split it in two columns, one containing the Nationality, which is the left side of the name before the underscore, and one containing the Year, right side of the underscore. I have tried with concat.split which should be the easiest way:I have a dataframe with around 1.5 million rows