The relationship between the White House and Congress on foreign policy has often been called "an invitation to struggle." The last two years of the Obama administration are shaping up as a classic example of that tension.

In one sense, that's healthy. The American system was deliberately designed to separate powers and balance competing institutions. But in another sense, it's very dangerous.

In a rapidly changing and interconnected world -- most people had never heard of ISIS a year ago -- only the president has the ability to lead the country's foreign relations with clarity and decisiveness. Another old adage is also true: The country cannot have 535 secretaries of state.

It was devious and damaging for House Republicans to invite Israeli Prime Minister Benjamin Netanyahu to address Congress on the subject of a nuclear deal with Iran without even telling the White House.

It was even more scurrilous for 47 Republican senators to sign a letter to the leadership in Tehran, warning that any agreement forged with Obama could be undone by the next president.

The dividing line between Congress and the president on international issues has always been murky. Wherever that line is, however, Republicans clearly crossed it. Twice.

Democrats were rightly outraged. They were correct to point out that if roles were reversed, and a Democratic Congress tried to vitiate the authority of a Republican president, the GOP would quickly cry treason.

But wait. The roles have been reversed. Many times. So Democratic tantrums, while justified, have to be tempered by a sense of history.

In the early '70s, Democrats pushed bills restricting the way a Republican president, Richard Nixon, could conduct the war in Vietnam. In 1973, Congress passed the War Powers Act -- over Nixon's veto -- that further limited a president's military options.

In the early 1980s, Democrats enacted the Boland Amendment, which barred another Republican president, Ronald Reagan, from aiding the Contra rebels in Nicaragua.

In 2002, 21 Democratic senators -- including Barack Obama -- voted against authorizing President Bush's invasion of Iraq, as did 126 Democratic House members.

So it's important to dissect what's really going on here. Republicans are partly motivated by a deep and abiding mistrust of Iran, which resumed talks about its nuclear future with six major powers in Switzerland this week. Senate leader Mitch McConnell was reflecting that hostility when he said on CNN: "Apparently the administration is on the cusp of entering a very bad deal with one of the worst regimes in the world that would allow them to continue to have their nuclear infrastructure."

Many Republicans see Obama as a home-grown version of the ayatollahs, especially since he's determined to use executive orders to circumvent Congress on such issues as immigration reform.

Democrats would certainly be justified in responding, "Get over it. Obama won twice. He's the only president we've got."

But Republicans won, too. They control both houses of Congress. And even some Democrats on Capitol Hill think the White House is not paying enough deference to the legislature's legitimate role in the "struggle" over foreign policy.

Sen. Bob Corker of Tennessee, the new chairman of the Foreign Relations Committee, has proposed a bill that would require the president to submit any deal he makes with Iran to a Congressional vote. The measure would also abrogate the president's right to waive the economic sanctions Congress previously imposed on Tehran.

The White House threatens to veto the measure, but since about a dozen Senate Democrats support it, sustaining a veto is not automatic. The White House also assures Corker and other lawmakers that Congress has "a role to play" in the Iranian talks, but they decline to define that role. And since they also insist that Congress cannot block or amend any deal that might be reached, it's hard to see what "role" Congress might play that's more than cosmetic.

During their last two years in office, all presidents turn to foreign policy -- partly to build a legacy, partly to escape a domestic arena where their power is steadily eroding. A continuing struggle with Congress is inevitable, and in order to avoid total paralysis, both sides have to recognize some basic truths.

There's only one president and one secretary of state, and they're both Democrats. But there's only one House speaker and one Senate majority leader, and they're both Republicans.

Both sides won elections. Both have been mandated -- by law and history -- to share power. If they don't use that power wisely, America's essential role in the world could suffer severely.