As helicopters buzzed overhead, Bulgaria's Prime Minister Boyko Borisov was in defiant form at weekend exercises on the border with Greece -- meant as a show of force to any would-be migrant considering an alternative way into the EU.

"More than 400 personnel from the army, police and gendarmerie will be permanently based here," Borisov declared. "Five hundred more can be mobilized within hours."

Exactly two years after Malaysia Airlines Flight MH370 disappeared with 239 people aboard, its whereabouts remain a mystery, with only one confirmed piece of wreckage found despite the most expensive search operation in history.

Here are some of the key questions still swirling around MH370:

Hillary Clinton is on a roll. If her candidacy ever looked in doubt to an insurgent Bernie Sanders, she's all but guaranteed the Democratic nomination -- thanks overwhelmingly to African Americans.

A month after her bruising defeat in New Hampshire, where Sanders won every category of voter except those older than 65 and earning more than $200,000 a year, Clinton has chalked up massive wins.

Brazilian President Dilma Rousseff will fight for her political life in Congress, the courts and streets this week, but her path to survival is getting ever narrower, analysts say.

Rousseff faces impeachment proceedings over alleged fiscal mismanagement, while the Supreme Court is considering possible campaign funding irregularities that could end up annulling her 2014 reelection.

When Boko Haram pledged allegiance to the Islamic State group in Iraq and Syria last year, there were fears the deadly insurgency in northeast Nigeria would take on an international dimension.

The world's deadliest designated terrorist organizations -- notorious for the indiscriminate slaughter of tens of thousands of people -- were now officially linked.

Weak oil prices pose a threat to Gulf Arab states' currency pegs against the dollar, but the energy-rich region is unlikely to abandon the policy yet, analysts say.

Bahrain, Oman, Qatar, Saudi Arabia and the United Arab Emirates all keep the values of their currencies fixed against the greenback, while Kuwait has a link to a basket of currencies including the dollar.

For most Russians, there's little not to like about their country's military operation in Syria.

The airstrikes have demonstrated Russia's might, turned the course of the war and made sure that Russia is once again treated as a world power on a par with the United States. And all at little cost.

Driving fearlessly under skies free of warplanes and bombs, taxi drivers in Syria's divided second city Aleppo are counting on a fragile truce to revive their trade.

One week into the ceasefire in parts of Syria, the country's iconic yellow taxis are slowly filling the rubble-strewn streets in eastern neighborhoods.

The migrant crisis poses possibly the most serious threat to the European Union's existence since the bloc's foundation as the ghosts of nationalism increasingly haunt the post-war dream of unity, political veterans warn.

While the EU has previously weathered storms ranging from the fall of the Berlin Wall to the eurozone debt crisis, they believe the huge influx of refugees and migrants taps into more dangerous currents that could yet sink the bloc.

Planning is at an advanced stage, special forces are already on the ground and air power assets are being moved within range: a new Western military intervention in Libya is edging ever closer.

But a long-anticipated move against offshoots of the Islamic State group remains on hold as long as Libya has not formed a unified government with the authority to ask for help to stem the extremist group's growth.