As tensions escalate over Ukraine, the U.S. military has deployed additional forces to reassure NATO allies rattled by Russia’s actions. But it remains unclear how much military help the United States can provide to Ukraine without triggering a direct conflict with Russia.

In a toe-to-toe conventional fight, experts say the U.S. military’s globe-spanning force would clobber the Russian army. But modern wars rarely involve such a match-up, and geography, politics and terrain inevitably give one side an advantage.

The United States and Russia differ profoundly in their views of the world and their roles in it. Capitalist America sees itself as a force for global stability and prosperity, while Russia’s centralized, authoritarian state seeks to limit Western power and increase Soviet domination.

In the past, the two countries found ways to work together, such as during the immediate aftermath of 9/11 when they cooperated on terrorism and Afghanistan; and in 2008-12, during Presidents Barack Obama’s “reset” with Dmitry Medvedev. But today, Washington and Moscow view each other as adversaries.

In the United States, three-quarters of Republicans and seven-in-ten Democrats say Russia is an enemy of the nation. This share has decreased slightly since last year, when majorities of liberal Democrats and moderate Republicans also expressed this view. Americans with at least a bachelor’s degree are more likely to consider Russia an enemy, while those without a college diploma are less inclined. This is a reflection of the growing divide in the country over how to respond to Russia’s aggressive actions in Ukraine and elsewhere, including its hacking and meddling in the 2016 U.S. election campaign.