The “frozen conflict” in East Ukraine is thawing out. This week, in the wake of a much-anticipated phone call between President Donald Trump and Russian President Vladimir Putin, fighting in the former Soviet Republic’s eastern districts erupted again. Since Sunday, eight Ukrainian soldiers have been killed with another 26 wounded. According to the government in Kiev, an unspecified number of civilians have also become casualties in the fighting. The often intense violence between Ukrainian troops and Russian-supported forces has, however, been met with stony silence from a distracted political class in the United States. It would be a grave mistake to ignore this slowly boiling conflict.

Both Kiev and Moscow blame one another for the recent surge in fighting, but that is an immaterial concern for Americans who want to see Russian aggression contained. From the Middle East to the frontiers of NATO, it isn’t Ukraine but Russia that has demonstrated a penchant for irredentism and expansionism. The West and the citizens of the United States should not overlook this war. It may be a prelude to something with more immediate repercussions for global peace and stability.

In the campaign, Donald Trump made no secret of his desire to cede to Moscow a sphere of unrivaled geopolitical influence in the Middle East and Europe. He even went so far as to entertain the prospect that he would not come to the aid of a NATO ally like Estonia if it invoked the Atlantic Alliance’s mutual defense provisions following a Russian attack.

The danger in these statements was not that Donald Trump would take office and immediately retrench from Europe and the Middle East, yielding to Putin something akin to a reconstituted Warsaw Pact—although Trump’s commitment to deferring to Putin’s objectives is a source of concern. The most urgent fear is that Trump’s statements gave Putin the idea that a Trump administration will provide him with new latitude. If Putin is under the impression that he will enjoy renewed freedom of action in what Moscow defines as its near abroad, he will likely test that presumption by probing the parameters of the West’s commitment to defend its interests. That probing may be ongoing right now in Eastern Ukraine.

If the Russian-backed offensive in Ukraine goes entirely ignored by Washington, Putin will have his answer. Moscow-backed separatists will get more aggressive, Russia will become more emboldened, and the next test may occur in a theater that Donald Trump cannot ignore. It may come from an incursion into Iraq at the invitation of Baghdad or the introduction of offensive intermediate-range nuclear missiles into the Russian enclave of Kaliningrad. It may come in the form of a cross-border incursion into a Baltic republic, and it wouldn’t be the first time something like that has happened.

If Russia made such a brazen assault on American interests, President Trump would soon learn that the institutional pressures on the American president to respond cannot be so easily ignored. The greatest risk is that Russia will miscalculate its way into an act of aggression to which the United States and the West must respond. Those are the kinds of calamities from which wars result.

In the Obama years, congressional Republicans made the strategically valid and morally righteous decision to hold the Democratic president firm to his commitment to the defense of America’s partners in Europe. Barack Obama had no desire to antagonize Russia in 2014, even after the invasion and annexation of Ukrainian territory on the Crimean Peninsula. He needed Russian cooperation to remove chemical weapons from Syria and to secure a nuclear accord with Iran. It was the GOP-led House that invited Ukrainian President Petro Poroshenko to address a joint session of Congress. It was a Republican-dominated House and Senate that approved lethal aid to Ukraine to fight its war with Russian proxies—a bill that was vetoed by Barack Obama. Will Republicans now abandon their virtuous defense of America’s commitment to the territorial integrity of Ukraine and the thwarting of Russian aggression merely because the president—this time, a Republican—is opposed? It would not just be a tragedy but a dereliction of their responsibilities to posterity if they did.

The war in Europe is again hot, and we ignore it at our own peril. If this is a prelude to another provocation somewhere else, somewhere the United States is not at liberty to simply ignore, we will regret this missed opportunity.