We study the vacuum structure of N f flavour two-dimensional QED with an arbitrary integer charge k. We find that the axial symmetry is spontaneously broken from Z kN f to Z N f due to the non-vanishing condensate of a flavour singlet operator, resulting in k degenerate vacua. An explicit construction of the k vacua is given by using a non-commutative algebra obtained as a central extension of the Z kN f discrete axial symmetry and Z k 1-form (center) symmetry, which represents the mixed 't Hooft anomaly between them.We then give a string theory realization of such a system with k = 2 and N f = 8 by putting an anti D-string in the vicinity of an orientifold O1 − -plane and study its dynamics using the twodimensional gauge theory realized on it. We calculate the potential between the anti D-string and the O1 − -plane and find repulsion in both weak and strong coupling regimes of the two-dimensional gauge theory, corresponding to long and short distances, respectively. We also calculate the potential for the (Q, −1)-string (the bound state of an anti D-string and Q fundamental strings) located close to the O1 − -plane. The result is non-perturbative in the string coupling. 4 dim massless QCD. † The case with N f = 1 is studied in [1], in which case the vacuum structure is similar to that of 4 dim N = 1 SU (N ) SYM, where U (1) A is broken to Z 2N by anomaly and further broken spontaneously to Z 2 , resulting in N vacua. * See [27] for the normal-ordering prescription. † See, e.g., [20] for a review. * These cusp singularities exist even for k = 1. The existence of the cusps can be understood from the mixed anomaly between the vector-like flavour symmetry SU (N f ) V /(Z N f ) V and the charge conjugation symmetry discussed in [21]. We thank the anonymous referee for pointing this out to us. † The overall factor in (2.71) is different from the expression for the energy density given in [28]. In [28], only the contribution from the fermion mass term is taken into account. We found that the kinetic term also has a contribution of the same order and included in (2.71). See Appendix B.2 for details.