WASHINGTON (AP) — The failure of last week's high-stakes diplomatic meetings to resolve escalating tensions over Ukraine has put Russia, the United States and its European allies in uncharted post-Cold War territory, posing significant challenges for the main players to avoid an outright and potentially disastrous confrontation.

Unlike previous disagreements that have arisen since the collapse of the Soviet Union, the current Ukraine crisis and seemingly insurmountable differences between Washington and Moscow carry real risks of debilitating economic warfare and military conflict that are exa...