Using 65 × 10 6 Υ ( 4 S ) → B ¯¯¯ B events collected with the BABAR detector at the PEP-II e + e − storage ring at the Stanford Linear Accelerator Center, we measure the color-favored branching fractions B ( ¯¯¯ B 0 → D + π − ) = ( 2.55 ± 0.05 ± 0.16 ) × 10 − 3 , B ( ¯¯¯ B 0 → D * + π − ) = ( 2.79 ± 0.08 ± 0.17 ) × 10 − 3 , B ( B − → D 0 π − ) = ( 4.90 ± 0.07 ± 0.22 ) × 10 − 3 and B ( B − → D * 0 π − ) = ( 5.52 ± 0.17 ± 0.42 ) × 10 − 3 , where the first error is statistical and the second is systematic. With these results and the current world average for the branching fraction for the color-suppressed decay ¯¯¯ B 0 → D ( * ) 0 π 0 , the cosines of the strong phase difference δ between the I = 1 / 2 and I = 3 / 2 isospin amplitudes are determined to be cos  δ = 0.872 + 0.008 + 0.031 − 0.007 − 0.029 for the ¯¯¯ B → D π process and cos  δ = 0.924 + 0.019 + 0.063 − 0.017 − 0.054 for the ¯¯¯ B → D ∗ π process. Under the isospin symmetry, the results for cos  δ suggest the presence of final-state interactions in the D π system.