We study dynamics of domain walls in pattern forming systems that are externally forced by a moving space-periodic modulation close to 2:1 spatial resonance. The motion of the forcing induces nongradient dynamics, while the wave number mismatch breaks explicitly the chiral symmetry of the domain walls. The combination of both effects yields an imperfect nonequilibrium Ising-Bloch bifurcation, where all kinks (including the Ising-like one) drift. Kink velocities and interactions are studied within the generic amplitude equation. For nonzero mismatch, a transition to traveling bound kink-antikink pairs and chaotic wave trains occurs. DOI: 10.1103/PhysRevLett.99.028302 PACS numbers: 82.40.Ck, 05.45.Yv, 47.54.ÿr The effects of external forcing on pattern forming systems exhibit fascinating nonlinear behavior from a fundamental point of view and at the same time provide a valuable tool for the control of pattern forming systems. While the cases of purely temporal [1][2][3] and spatial forcing [4 -6] have been considered for many years, the spatiotemporal modulation of control parameters has been introduced only recently [7][8][9][10][11][12][13]. Simultaneously, both fundamental questions [14] and interesting applications of pattern control have arisen for possible information processing devices based on nonequilibrium patterns [15,16].In the simplest case of a spatiotemporal forcing, using the form of a traveling wave, one allows for a periodic dependence on both space and time. The consequences of spatial resonance of such a forcing with a Turing-like mode were studied in terms of the frequency, or velocity, !, of the traveling-wave forcing and the deviation q from exact spatial resonance. A central finding was the occurrence of kinks or domain walls [7], which, similar to the case of purely spatial forcing [5], mediate the competition between the inherent and imposed wavelength.In this Letter we study how a mismatch q and the motion of the forcing affect domain walls. We show that a combination of both effects, studied in the exemplary case of 2:1 resonance, provides a new scenario of complex spatiotemporal dynamics. Beginning with exact spatial resonance, q 0, an illuminating analogy with the resonance of a subharmonic temporal forcing of an extended oscillatory system appears. For the latter system it is well known that the resonance generates stable walls and a transition between Ising and Bloch walls. The motion of the forcing pattern -playing the role of the frequency detuning in oscillatory systems-endows the system with nongradient dynamics. This transition is called nonequilibrium IsingBloch (NIB) transition [17][18][19][20][21].The effects of a spatial resonance mismatch are more difficult to analyze. While for large bifurcation parameter (or, equivalently, for small forcing) the effect of the mismatch becomes negligible and a phase approximation can be used [11], we here focus on intermediate values of . We employ numerical continuation methods to calculate kink solutions and show that the mismatch q...