The contingency feared by Western planners is Russia invading while the US is unable or unwilling to come to Poland's aid. The two most obvious scenarios are Trump getting elected and withdrawing the US from NATO (either officially, or by using his authority as commander in chief to refuse to deploy US forces in support of Article V), or US forces being tied up in a war with China. European nations would probably help, but European militaries don't have the same expeditionary capabilities as the US so they'd probably be limited in how much they could practically do.