How exhausting it was to argue head-on against the position 'Don't just follow America's lead!'
Literal
America [gen-の] coattails [to-に] ride [don't-な] [quotative-という] line [to-に] front [from-から] rebut [nominalizer-の] [subj-が], how-much exhausting thing [copula-だったことか].
尻馬に乗る 'to ride someone's coattails / to jump on the bandwagon' literally 'to ride the rear horse.' セリフ 'line / dialogue' is used for someone's stated position. 正面から 'head-on / directly from the front.' どれほど~ことか 'how X it was!' is a rhetorical exclamation of degree. しんどい 'exhausting / tough' is a Kansai-origin word now used nationwide.