  A nonperturbative approach to QCD describing confinement and chiral symmetry
breaking is discussed. It is based on the path integral representation of
Green's function of quarks and leads to the QCD string theory. The effective
actions for mesons and baryons in the external uniform static electromagnetic
fields are obtained. The area law of the Wilson loop integral, the
approximation of the Nambu-Goto straight-line string, and the asymmetric
quark-diquark structure of nucleons are used to simplify the problem. The
spin-orbit and spin-spin interactions of quarks are treated as a perturbation.
Using the virial theorem we estimate the mean radii of hadrons in terms of the
string tension and the Airy function zeros. On the basis of the perturbation
theory in small external electromagnetic fields we derive the electromagnetic
polarizabilities of nucleons. The electric and diamagnetic polarizabilities of
a proton are $\bar{\alpha}_p= 10\times 10^{-4} fm^3$, $\beta_p^{dia}=-8\times
10^{-4} fm^3$ and for a neutron we find $\bar{\alpha}_n=4.2\times 10^{-4} fm^3
$, $\beta_n^{dia}= -5.4\times 10^{-4} fm^3$. Using the $\Delta$ contribution to
the paramagnetic polarizability of the nucleons, reasonable values of the
magnetic polarizabilities $\bar{\beta}_p=(5\pm 3)\times 10^{-4} fm^3$,
$\bar{\beta}_n=(7.6\pm 3)\times 10^{-4} fm^3$ are estimated.
