President Obama has committed to working with U.S. trade partners to avoid “escalating protectionism.” He is wise to do so. As never before, U.S. national security requires a commitment to open trade.

President Obama and his foreign counterparts should reflect on the lessons of the 1930s—and the insights of Cordell Hull. The longest-serving secretary of state in American history (1933–1944), Hull helped guide the United States through the Depression and World War II. He also understood a fundamental truth: “When goods move, soldiers don’t.”

In the 1930s, global recession had catastrophic political consequences—in part because policymakers took exactly the wrong approach. Starting with America’s own Smoot Hawley Tariff of 1930, the world’s major trading nations tried to insulate themselves by adopting inward looking protectionist and discriminatory policies. The result was a vicious, self-defeating cycle of tit-for-tat retaliation. As states took refuge in prohibitive tariffs, import quotas, export subsidies and competitive devaluations, international commerce devolved into a desperate competition for dwindling markets. Between 1929 and 1933, the value of world trade plummeted from $50 billion to $15 billion. Global economic activity went into a death spiral, exacerbating the depth and length of the Great Depression.

The economic consequences of protectionism were bad enough. The political consequences were worse. As Hull recognized, global economic fragmentation lowered standards of living, drove unemployment higher and increased poverty—accentuating social upheaval and leaving destitute populations “easy prey to dictators and desperadoes.” The rise of Nazism in Germany, fascism in Italy and militarism in Japan is impossible to divorce from the economic turmoil, which allowed demagogic leaders to mobilize support among alienated masses nursing nationalist grievances.

Open economic warfare poisoned the diplomatic climate and exacerbated great power rivalries, raising, in Hull’s view, “constant temptation to use force, or threat of force, to obtain what could have been got through normal processes of trade.” Assistant Secretary William Clayton agreed: “Nations which act as enemies in the marketplace cannot long be friends at the council table.”

This is what makes growing protectionism and discrimination among the world’s major trading powers today so alarming. In 2008 world trade declined for the first time since 1982. And despite their pledges, seventeen G-20 members have adopted significant trade restrictions. “Buy American” provisions in the U.S. stimulus package have been matched by similar measures elsewhere, with the EU ambassador to Washington declaring that “Nobody will take this lying down.” Brussels has resumed export subsidies to EU dairy farmers and restricted imports from the United States and China. Meanwhile, India is threatening new tariffs on steel imports and cars; Russia has enacted some thirty new tariffs and export subsidies. In a sign of the global mood, WTO antidumping cases are up 40 percent since last year. Even less blatant forms of economic nationalism, such as banks restricting lending to “safer” domestic companies, risk shutting down global capital flows and exacerbating the current crisis.

If unchecked, such economic nationalism could raise diplomatic tensions among the world’s major powers. At particular risk are U.S. relations with China, Washington’s most important bilateral interlocutor in the twenty-first century. China has called the “Buy American” provisions “poison”—not exactly how the Obama administration wants to start off the relationship. U.S. Treasury Secretary Timothy Geithner’s ill-timed comments about China’s currency “manipulation” and his promise of an “aggressive” U.S. response were not especially helpful either, nor is Congress’ preoccupation with “unfair” Chinese trade and currency practices. For its part, Beijing has responded to the global slump by rolling back some of the liberalizing reforms introduced over the past thirty years. Such practices, including state subsidies, collide with the spirit and sometimes the law of open trade.

The Obama administration must find common ground with Beijing on a coordinated response, or risk retaliatory protectionism that could severely damage both economies and escalate into political confrontation. A trade war is the last thing the United States needs, given that China holds $1 trillion of our debt and will be critical to solving flashpoints ranging from Iran to North Korea.

In the 1930s, authoritarian great-power governments responded to the global downturn by adopting more nationalistic and aggressive policies. Today, the economic crisis may well fuel rising nationalism and regional assertiveness in emerging countries. Russia is a case in point. Although some predict that the economic crisis will temper Moscow’s international ambitions, evidence for such geopolitical modesty is slim to date. Neither the collapse of its stock market nor the decline in oil prices has kept Russia from flexing its muscles from Ukraine to Kyrgyzstan. While some expect the economic crisis to challenge Putin’s grip on power, there is no guarantee that Washington will find any successor regime less nationalistic and aggressive.

Beyond generating great power antagonism, misguided protectionism could also exacerbate political upheaval in the developing world. As Director of National Intelligence Dennis Blair recently testified, the downturn has already aggravated political instability in a quarter of the world’s nations. In many emerging countries, including important players like South Africa, Ukraine and Mexico, political stability rests on a precarious balance. Protectionist policies could well push developing economies and emerging market exporters over the edge. In Pakistan, a protracted economic crisis could precipitate the collapse of the regime and fragmentation of the state. No surprise, then, that President Obama is the first U.S. president to receive a daily economic intelligence briefing, distilling the security implications of the global crisis.

What guidance might Cordell Hull give to today’s policymakers? To avoid a protectionist spiral and its political spillovers, the United States must spearhead multilateral trade liberalization involving all major developed and developing countries.

Starting in 1934, Hull used new authorities under the Reciprocal Trade Agreements Act (RTAA) to launch a trade liberalization campaign unprecedented in American history. By the outbreak of World War II, the United States had reached RTAA agreements with twenty nations.

Hull’s valiant efforts began too late to prevent the worst damage and avert world war, and he undoubtedly exaggerated the potential for trade liberalization to cut the economic taproots of global conflict. But as Hull proudly observed, the United States did not wage war against a single country with which it had signed an RTAA agreement, and most of its RTAA partners joined in resisting the Axis. “The political line-up followed the economic line-up.” More importantly, Hull’s logic helped underpin the Roosevelt administration’s postwar vision of an open multilateral system of trade and payments, governed by new international institutions like the General Agreement on Tariffs and Trade, IMF and World Bank.

When the G-20 reconvenes in April in London, its members will have another chance to get it right. At all costs, they must avoid the infamy of a previous London Economic Conference—in 1933—when the United States sabotaged a coordinated response to the Great Depression. Avoiding a similarly disastrous result today will require President Obama to exercise far-sighted leadership in the manner of a modern-day Cordell Hull.