All you did was link to the main page of a wikipedia article and copy and paste the first sentence. Your response is so lazy, it doesn't even deserve a response, but I'm putting this out here for the benefit of the general public:
https://www.icrc.org/en/article/grave-breaches-defined-genev...
GC 4 Art. 147.
"Grave breaches to which the preceding Article relates shall be those involving any of the following acts, if committed against persons or property protected by the present Convention: wilful killing, torture or inhuman treatment, including biological experiments, wilfully causing great suffering or serious injury to body or health, UNLAWFUL DEPORTATION OR TRANSFER OR CONFINEMENT OF A PROTECTED PERSON, compelling a protected person to serve in the forces of a hostile Power, or wilfully depriving a protected person of the rights of fair and regular trial prescribed in the present Convention, taking of hostages and extensive destruction and appropriation of property, not justified by military necessity and carried out unlawfully and wantonly."
1. Foreign heads of state are definitely protected persons.
2. Foreign heads of state transiting to and from diplomatic meetings are engaged in a protected activity.
3. If these laws apply between enemy nations engaged in declared war, they are even more applicable to countries at peace with one another.
It turns out I'm even more right that I initially thought: this was not only a breach of the spirit of the Geneva Conventions, it was also a breach of the very letter of the law! Regardless, someone doesn't understand the purpose of the Geneva Conventions in the first place, so I'll elaborate...
Edward Snowden himself is irrelevant, it doesn't matter if Osama Bin Laden was on that plane. The fact is that the US and its allies used deception to illegally ground a diplomatic flight, detain a foreign head of state, and engage in an illegal search and seizure.
Furthermore, whether or not the countries involved were even at war is irrelevant. The purpose of the Geneva Conventions are to maintain a minimum set of international ethics that make diplomacy safe for diplomats. If a foreign head of state can be detained or imprisoned, and if his property can be searched or seized, then diplomatic negotiations for anything are now impossible.
It doesn't matter if the reasons for breaking these rules are justifiable or not, the fact is that you're not trustworthy even in a basic capacity that allows for diplomatic negotiation. You're in the same perfidious bucket as Japanese Emperor Hirohito, Saddam Hussein, or Ruhollah Khomeini (Iranian Hostage Crisis).
"Build your opponent a golden bridge to retreat across."
-Sun Tzu
P.S. The Vienna Convention on Diplomatic Relations explicitly forbids detaining diplomats. See articles 27 and 29:
https://legal.un.org/ilc/texts/instruments/english/conventio...
YOU LOSE! YOU GET NOTHING! GOOD DAY, SIR!