The United States and its allies edged closer Tuesday to launching military strikes against Syria over alleged deadly chemical attacks, while Damascus has vowed to defend itself.
Fierce opposition from Syria ally Russia, one of the five veto-wielding permanent members of the UN Security Council, means that any action may have to be taken without UN approval.
Here are some examples of international military strikes launched either without UN backing, or with a UN mandate that was later disputed.
Operation Allied Force
On March 24, 1999, NATO forces launched air strikes on Yugoslavia that lasted until June 11. They were ordered after peace talks between officials in Belgrade and representatives of the ethnic Albanian majority in Kosovo broke down.
Russia and China threatened to veto any UN resolution that authorised a military intervention.
The attacks followed Yugoslav strongman Slobodan Milosevic's repeated refusal to accept an accord hammered out in February, and aimed to protect ethnic Albanians from Yugoslav forces made up mainly of ethnic Serbs.
The action was also a response to a fierce Yugoslav offensive against the separatist Kosovo Liberation Army (KLA).
Around 600 aircraft from 13 countries took part in daily air strikes on targets in the Yugoslav provinces of Kosovo, Montenegro and Serbia.
On June 10, 1999, Serb-dominated Yugoslav forces began to withdraw from Kosovo, which was then placed under UN administration.
Operation Iraqi Liberation, later Operation Iraqi Freedom
In March 2003, Britain, Spain and the United States sought a UN resolution authorising a military offensive in Iraq, which was suspected of holding stocks of arms of mass destruction.
The proposal was withdrawn when it became clear there was no consensus behind it, with France, Germany and Russia in particular voicing opposition.
After giving Iraqi president Saddam Hussein a final ultimatum, the US government of George W. Bush decided to act without a UN mandate.
On March 20, 2003, the US launched its invasion with air strikes on Baghdad and British and US ground troops in the south of the country.
Operation Iraqi Freedom resulted in the overthrow of Saddam, who was captured, tried in an Iraqi court, and hanged on December 30, 2006.
In May 2003, the US-led coalition comprised around 150,000 US troops along with 23,000 others from about 40 countries.
On October 16, 2003, UN resolution 1511 approved a US-led multinational force to maintain security in the country.
In December 2011, almost all US troops were withdrawn.
Operation Unified Protector
On March 17, 2011, UN Security Council Resolution 1973 authorised member states "to take all necessary measures... to protect civilians and civilian populated areas under threat of attack" by forces loyal to Libyan leader Moamer Kadhafi.
He was battling an armed revolt that had begun the previous month.
Operation Unified Protector began on March 19, and was led by Britain and France with active support from the United States. NATO took command of the mission on March 31. In all, 18 countries took part in the operation.
In June 2011, China and Russia accused the NATO-led coalition of going beyond the UN mandate by seeking the overthrow of Kadhafi, which France acknowledged.
On October 20, Kadhafi was cornered and killed during an assault on his hometown of Sirte, two months after Tripoli fell to rebel forces thanks in large part to NATO air cover.