An old truism recommends keeping your friends close and your enemies closer. But how to tell the two apart?

Alliances in international politics are at best a necessary evil, somewhat analogous to government in liberal political philosophy. For a regional hegemon with global interests, like the United States, allies are particularly indispensable, given Washington’s need to project power globally.

That fact is cold comfort for the diplomats and military officers tasked with maintaining them, as even the best allies are a never-ending source of migraines and anguish. Many would contend that America has no greater friend than Israel. And yet, Israel is a counterintelligence nightmare with a habit of announcing settlement expansions at particularly inopportune times for U.S. officials.

It is hardly an anomaly in this regard. France, America’s oldest ally, was constantly at odds with the United States during the Cold War, criticized America as a hyperpower in the decade after it and led global opposition to the invasion of Iraq in 2003. Similarly, the U.S.-Japanese alliance may be the foundation of America’s alliance system in Asia. Still, despite initially welcoming his election, U.S. officials have been dismayed by Japanese premier Shinzo Abe's historical analysis and field trips to the Yasukuni Shrine.

No U.S. ally is perfect. But five alliances of convenience in particular stand out. (Note: the list is not limited to formal treaty allies.)

1. Imperial Japan

As most Japanese celebrated the success of Pearl Harbor, the architect of the attack, Admiral Isoroku Yamamoto, reflected ominously, “I fear all we have done is to awaken a sleeping giant and filled him with a terrible resolve."

If only U.S. officials had demonstrated such prescience nearly a century before when they sent Commodore Matthew Perry to forcibly open up the hyperisolationist country. Although Perry was successful in his immediate objective, the United States ultimately got more than it bargained for in the exchange.

After shedding their initial reluctance, Japanese leaders embraced modernization with a fervor nearly unparalleled in human history. Underlying this drive was a desire to transform Japan into a great power so that it could never again be bullied by Western powers.

Although the bilateral relationship was never free of tension, particularly over China and immigration issues, the United States initially found much to like in an increasingly powerful Japan. For example, Tokyo joined American and European powers in helping to suppress the Boxer Rebellion in China. Even before then, Theodore Roosevelt used a rising Japan as a check against Russian power in Asia. Japan was also valuable in commandeering Germany’s Pacific holdings during WWI. Following the war, and despite continued tensions, Japan and the United States signed a number of important agreements at the Washington Conference of 1922.

Of course, all of this was more than outweighed by what followed, as tensions over China and Asia greatly intensified. This ultimately culminated in the attack on Pearl Harbor that killed over 2,000 Americans and wounded more than a thousand others. Imperial Japan followed up the surprise attack by conquering much of the Asia-Pacific, including the U.S.-controlled Philippines. It would rule over these territories with barbaric savagery.

Removing Japan from these lands—which fell primarily to the United States—proved no easy task. Imperial Japanese soldiers surrendered at appallingly low rates. According to some sources, only one to three percent of Japanese forces surrendered throughout the war, and only one third of these troops actually wanted to surrender (the rest were too sick or wounded to commit suicide or continue fighting.) As a result, most estimates suggest that America’s casualty rate in the Pacific theater was about three and a half times larger than in Europe.

2. The Soviet Union

A case could be made that the Soviet Union was actually one of America's best allies to date. After all, while the United States has struggled to get even the most marginal military contributions from its NATO allies, the Soviet Union tied down about 70 percent of Nazi forces during WWII and accounted for around 75 percent of German military casualties.

Nonetheless, no list would be complete without the Soviet Union. Even FDR, one of the earliest and most ardent supporters of the alliance, compared partnering with the Soviet Union to holding hands with the devil.

To begin with, as large as the Soviet Union’s contribution to the allied victory was, Moscow hardly offered up this support enthusiastically. To the contrary, Stalin had originally allied with Germany in the hope that France and England would bear the burden of defeating Hitler. It was only when the Führer invaded the Soviet Union that Stalin joined the fight against the hated fascists. Even still, throughout the war, Stalin constantly (and understandably) demanded the United States and England open up a second European front immediately to relieve the Red Army. FDR and Churchill (just as understandably) demurred until the Red Army was on its way to Berlin.

In other words, the Soviet Union made its enormous contribution to the war effort only because it had no choice. Moscow also extracted a heavy price (at least in America’s eyes) for its role during the war by swallowing up most of Eastern and Central Europe. This set the stage for a global rivalry between the two superpowers that lasted almost half a century. Talk about buyer’s remorse!

3. Saddam Hussein’s Iraq

If Saddam Hussein wasn’t quite the monster that Stalin or Hitler was, it wasn’t for lack of ambition. During his quarter century in power, Hussein ruled with a brutality uninhibited by moral conscience. His ruthlessness extended beyond Iraq’s borders, as he frequently invaded unsuspecting neighbors. And when it came to using chemical weapons, Saddam did not distinguish between internal and external enemies.

It was therefore fitting that after taking power, Saddam continued to ally with the Soviet Union and have no relations with the United States. Even after the Iran-Iraq War first began, the United States adopted a policy of strict neutrality in a conflict pitting two countries in which it maintained no diplomatic relations against one another.

Ultimately, however, America’s disdain for revolutionary Iran—still fresh from the hostage crisis—allowed the Iraqi strongman to curry favor with Washington. When, in the spring of 1982, Iranian forces threatened to overrun Basra, the Reagan administration paved the way for supporting Saddam by removing Iraq from the list of state sponsors of terrorism. The following year, the U.S. president issued a classified National Security Decision Directive that pledged to do "whatever was necessary and legal" to prevent Iraq from losing the conflict. Soon thereafter, Iraq and the United States restored diplomatic relations during a trip to Washington by Iraqi foreign minister (and Saddam’s right-hand man) Tariq Aziz, which included meetings with President Reagan and Secretary of State George Shultz.

Even before that time, in 1982, the United States had begun to furnish Iraq with various kinds of assistance to bolster its war effort. This support would only grow more expansive throughout the war. As one senior official at the time later recounted:

The United States actively supported the Iraqi war effort by supplying the Iraqis with billions of dollars of credits, by providing U.S. military intelligence and advice to the Iraqis, and by closely monitoring third country arms sales to Iraq to make sure that Iraq had the military weaponry required.

This included “strategic operational military” intelligence on Iranian troop positions, which the Iraqi army used to greatly enhance the effectiveness of its chemical weapon attacks, as well as “cluster bombs and anti-armor penetrators,” which CIA Director William Casey termed “force-multipliers.”

Of course, the United States would come to regret this cooperation nearly immediately after the war ended, when Saddam invaded Kuwait in 1990. Although the United States and its allies scored a decisive victory against Iraqi forces in the first Gulf War, Saddam would continue to haunt U.S. officials through the 2003 invasion of Iraq. Even the Clinton administration called Saddam and like-minded dictators ”the greatest security threat we face.” And, of course, the decision to remove Saddam proved beyond costly for the United States (to say nothing of Iraq and the greater region). It was therefore something of an understatement when the former deputy U.S. ambassador to Iraq later conceded of cooperating with Saddam: “History will demonstrate that this was a miscalculation."

4. Pakistan

To defeat Hitler in WWII, FDR was willing to hold hands with the devil. To combat Al Qaeda after 9/11, the United States literally partnered with the “ally from hell.”

Truth be told, America’s relationship with Pakistan long predated the 9/11 terrorist attacks. In fact, the United States was one of the first countries to formally recognize and establish ties with Pakistan in the late 1940s, and Washington made Islamabad an integral part of both its Central and Southeast Asia Treaty Organizations the following decade.

This longevity should not obscure the fact that the bilateral relationship has at best been a dysfunctional marriage.

Even in the brief moments when Pakistan has been valuable to the United States, it has undercut its case with serious transgressions. For example, during the 1970s, Pakistan helped facilitate America’s rapprochement with China while simultaneously committing genocide against the Bengalis in what was then Eastern Pakistan. Similarly, during the 1980s, Pakistan was instrumental in helping the United States and its allies funnel aid to the Afghan Mujahideen resisting the Soviet occupation of their country. At the same time, Islamabad used this support to force Washington to tacitly accept Pakistan’s burgeoning nuclear-weapons program.