NATO flags flutter during the final day of NATO Saber Strike exercises in Orzysz, Poland, on June 16. President  Trump is expected to make a speech in Warsaw’s Krasinski Square on Thursday, then head to Germany to attend the G-20 Summit on Friday and Saturday. (Ints Kalnins/Reuters)

President Trump’s second foreign trip kicks off today in Warsaw — the first leg of a trip that includes the G-20 Summit in Hamburg  and a stop in Paris for Bastille Day. The trip comes in the wake of the turmoil from the president’s European trip to the G-7 meeting in May, with the additional drama of the first Trump-Putin bilateral meeting.

But for  Trump and for the United States, the Warsaw stop will present a set of modern-day challenges with historical echoes. On the one hand, Trump probably will have his most favorable meetings in Poland. Warsaw’s right-wing government and anti-immigration stance, among other things, are more in line with his administration’s anti-internationalist stance.

On the other hand, Poland — like much of Europe — will also be looking for Trump to put European allies at ease, and make a strong U.S. commitment to NATO’s Article 5 treaty. Poland, along with the NATO member states bordering Russia, is fearful of Russia’s recent aggression spreading westward.

Poland and Estonia are two of only five NATO members that meet the target of spending 2 percent of gross domestic product  on defense, a commitment NATO members agreed to work toward after the 2014 Wales Summit. Three other nations in the region, Romania, Latvia and Lithuania, are set to join this list by next year.

Article 5 is the glue holding NATO together

What all of these countries want to hear is a firm statement on Article 5 of the NATO treaty — which simply stipulates that an attack on one alliance nation is an attack on them all. This is the core of the NATO alliance, and U.S. adherence to Article 5 dominates alliance members’ calculations, especially in Eastern Europe. Although Trump pledged U.S. adherence to Article 5 during a June news conference with the Romanian president, many in the alliance remain uncertain, given the president’s failure to make a public commitment during his speech to fellow NATO leaders in May.

Collective defense was the core of the NATO alliance’s formation and credibility in 1949, and it remains so. As a crucial first step in NATO’s creation — and a prerequisite as far as the United States was concerned — Britain, France, Belgium, the Netherlands and Luxembourg proved that they could come together for collective defense in the 1948 Brussels Treaty. To solidify a credible deterrent to the Soviet Union, the defense pact needed to expand to include the United States.

The biggest hurdle for the Truman administration at the time was overcoming a historical antipathy against alliances to create the first “entangling alliance” since the 1778 treaty with France. Realizing what was at stake in the growing Cold War, the administration worked across the political aisle to get key Republicans on board, most notably Sen. Arthur Vandenberg (R-Mich). In short, Vandenberg crafted the requisite legislation that would allow the United States to bind itself to the “progressive development of regional and other collective self-defense.”

As NATO historian Stanley R. Sloan points out, “Today, the collective defense commitment still endows the North Atlantic Treaty with special meaning. It is a potential deterrent against would-be enemies of the allies and a source of reassurance should future threats develop.”

Historically, Poland could use some reassurance

Poland hasn’t had the best of luck controlling its sovereignty over the past two centuries. It was partitioned between regional powers in the late 1700s and gained independence only in 1918. As we know from more recent history, that didn’t last long.

The Nazi-Soviet Non-Aggression Pact of August 1939 divided Poland between Hitler’s Germany and Stalin’s Soviet Union. On Sept. 1, 1939, the German invasion of Poland launched World War II. Berlin eventually reneged on the pact and invaded the Soviet Union in the summer of 1941. The Poles, especially Polish Jews, would suffer some of the worst atrocities of the war.

The Soviet Red Army’s liberation of Poland in 1945 and the conclusion of World War II didn’t give Warsaw much of a break. Stalin sought a sphere of influence in Eastern Europe to act as a buffer between himself and the West, the direction from which Russia’s adversaries had come twice in the past 30 years. Against American protests, the Soviets installed a Moscow-friendly communist government in Warsaw, ushering in close to 45 years of Soviet dominance. Warsaw would also become the namesake for the pro-Soviet alliance system — the Warsaw Pact — created in 1955 to become NATO’s counterfoil.

NATO’s front lines have shifted

During the Cold War, if a “hot war” between NATO and the Warsaw Pact was going to begin, the chances were it would begin in a divided Berlin. For more than 40 years, the two sides stared at one another across dividing lines with names such as “Checkpoint Charlie” — and weathered a major crisis from 1958 to 1961. The Berlin Wall’s fall in 1989, Germany’s unification and the collapse of the Soviet Union in 1991 ended the Cold War and nightmares of Soviet tanks crossing into Western Europe.

In the past 25 years, NATO expanded eastward to include former Warsaw Pact members such as Poland, as well as former Soviet states. As tensions with a resurgent Russia have risen, many of these states worry that, as in Ukraine and Crimea, the Russians will find a pretext to initiate a “hybrid war” in the region to regain lost influence and control. This is why such a large portion of Russia’s neighbors in Europe spend at least 2 percent of GDP on defense, or will by 2018.

As the Monkey Cage has noted, NATO stepped up its involvement in Eastern Europe in response to Russia’s actions in Ukraine, including Obama administration-ordered troop increases in Poland and other Eastern European nations. Poland and its Baltic neighbors will be looking for Trump to give strong assurances in a major speech he will deliver in Poland. Media reports after Trump’s May speech at NATO headquarters note that it appears the president intentionally removed a sentence reaffirming U.S. adherence to Article 5.

According to national security adviser H.R. McMaster, the president “will reiterate … America’s commitment to NATO’s common defense” this week in Poland. Whether this is a formal adherence to Article 5, a common understanding of the threat Russia poses to the region, or continued backing of American forces in the Baltics and Poland remains to be seen.

Kelly M. McFarland is a U.S. diplomatic historian and director of programs and research at Georgetown University’s Institute for the Study of Diplomacy and an adjunct professor in the Walsh School of Foreign Service.