As secretary of state, Hillary Clinton basked in a diplomatic "Moscow Spring," seizing on Vladimir Putin's break from the presidency to help seal a nuclear arms-control treaty and secure Russia's acquiescence to a NATO-led military intervention in Libya. But when Putin returned to the top job, things changed.

Clinton, the Democratic presidential nominee, has vowed to stand up to Putin if elected, drawing on her four years of ups and downs as the public face of President Barack Obama's first-term "reset" with Russia. By comparison, her Republican opponent, Donald Trump, has rung alarm bells in Washington and Europe with his overtures to the authoritarian Russian leader.

But Clinton's wrangles with Russia led to mixed results. And her fortunes dipped dramatically after Putin replaced Dmitry Medvedev as president in May 2012.

Just weeks later, Russia outmaneuvered her in negotiations over a complicated Syria peace plan, dealing her what was arguably her worst diplomatic defeat. While Clinton hailed it as a triumph, the war only escalated. And while her aides still insist she came out on top, the blueprint effectively gave Syria's Moscow-backed president, Bashar Assad, a veto over any transition government, hampering all mediation efforts still.

"There is no doubt that when Putin came back in and said he was going to be president, that did change the relationship," Clinton said in a Democratic debate last year. "We have to stand up to his bullying and specifically, in Syria it is important."

Clinton's history with Russia is significant given the surprising role Russia has played in the U.S. presidential campaign.

Clinton and her supporters say she would be far tougher on Moscow than Trump, whose unusual foreign policy statements include musings about NATO's relevance and suggestions that he could accept Russia's annexation of Ukraine's Crimea region. Russia's reported hacking of Democratic Party email accounts also has led to charges that Putin's intelligence services are meddling in the election, and Trump aided that perception by publicly encouraging Russia to find and release more of her emails.

Clinton's first encounters in Russian diplomacy began on much more hopeful note. Meeting Foreign Minister Sergey Lavrov in 2009, Clinton initiated the effort to repair years of bitter relations, punctuated by a Russian war with neighboring Georgia a year earlier. Offering a large red reset button, Clinton outlined a broad agenda of cooperation.

The new policy paid dividends.

With Putin focused on domestic matters during a four-year stint as prime minister, Medvedev opened up a new corridor for U.S. forces and materiel heading to Afghanistan. The two nations sealed their most ambitious arms control pact in a generation. Washington and Moscow united on new Iran sanctions. Years of trade negotiations culminated in Russia's entry into the World Trade Organization.

But it was perhaps Clinton's unlikeliest diplomatic breakthrough that began the downward spiral: Libya.

As America's European allies sought a military intervention against Libyan dictator Moammar Gadhafi, Clinton played the role of skeptic, refusing to jump aboard. When she finally did, it proved critical in persuading Russia to abstain. The rebels overthrew Gadhafi five months later.

Returning as president in May 2012, Putin was immediately confronted with Syria's Libya-like escalation from Arab Spring protests to full-scale civil war. He played his cards differently than Medvedev, hinting to Obama that he could drop his support for the Syrian leader while shielding Assad from any U.N. pressure or foreign action that might chase him from power.

Seeking Russia's cooperation, Obama and Clinton avoided any talk that might threaten Russian equities in Syria, including a large naval base there. Their message was clear: The U.S. wouldn't try to pull a future post-Assad government out of Moscow's orbit.

It didn't matter. When the U.N. proposed a peace plan that involved ushering Assad out of power and included penalties for noncompliance, the Russians balked. Faced with stalemate, the U.S. and Russia arrived at a formula for a new government comprised of individuals chosen by the "mutual consent" of Assad and the opposition.

Although Clinton claimed credit for the June 30, 2012, compromise in Geneva, it appeared to be Russia's objective all along.

Assad is still president.

Clinton has acknowledged her frustration with an increasingly hostile Russia on Syria and other matters as her time in office wound down.

In her final months, Russia ordered the end of all U.S. Agency for International Development programs in the country. It approved a new law constraining the work of Russian and foreign non-governmental organizations. It banned U.S. adoptions of Russian children.

In December 2012, Clinton accused Putin of trying to "re-Sovietize" its region. And just before leaving, she wrote a memo to Obama urging him to finally suspend a reset that ended once and for all with Russia's military incursions in Ukraine and annexation of Crimea in 2014 — well after Clinton had left government.

"Strength and resolve were the only language Putin would understand," Clinton wrote in her memoir "Hard Choices," published shortly afterward.

It's a lesson she could say she learned firsthand.