If Obama won't back up Ukraine against a clear act of war, after the US signed a treaty to do so to do so in 1994
The Budapest Memorandum on Security Assurances of December 1994 provided assurances that Ukraine's territory would be respected by the signatories (US, UK, Russia) in exchange for Ukraine giving up its nuclear arsenal. In the event that nuclear weapons are used against Ukraine, UN Security Council action is required. Given a territory violation, at most, the treaty could be used as justification for any US or UK actions taken to uphold Ukraine's territorial sovereignty, but as far as I am aware it does not bind any nation to take such actions. It seems pretty clear that Russia has violated this treaty. I would argue that Russia may have violated the treaty even before putting troops in Crimea through the use of natural gas economic blackmail to stop Ukraine's negotiations with the EU.
You are clearly bothered by U.S. inaction in Ukraine. What would you recommend that the U.S. do that it is not doing now? Threaten nuclear war? Invade Ukraine to repulse the Russian troops?
why in the world would anyone think that Obama would back Israel in case of a war?
This may be a fair question to ask, but I think it's weakened when you tie it to the situation in Ukraine, because the U.S. hasn't failed to honor the 1994 treaty.
Perhaps Israel should re-orient itself towards other countries that have similar interests in fighting the millitant spread of Islam, such as Russia and China.
Do you think Russia or China would support Israel over Iran if G-d forbid there was war?