  Spatiotemporal chaos arising from the competition between
sine-Gordon-breather and kink-antikink-pair solitons by reshaping an ac force
is demonstrated. After introducing soliton collective coordinates, Melnikov's
method is applied to the resulting effective equation of motion to estimate the
parameter-space regions of the ac force where homoclinic bifurcations are
induced. The analysis reveals that the chaos-order threshold exhibits
sensitivity to small changes in the force shape. Computer simulations of the
sine-Gordon system show good agreement with these theoretical predictions.
