Compression members, such as columns, are mainly subjected to axial forces. The principal stress in a compression member is therefore the normal stress,

The failure of a short compression member resulting from the compression axial force looks like,

However, when a compression member becomes longer, the role of the geometry and stiffness (Young's modulus) becomes more and more important. For a long (slender) column, buckling occurs way before the normal stress reaches the strength of the column material. For example, pushing on the ends of a business card or bookmark can easily reproduce the buckling.