I know that my title is an over-simplification, but it seems to me that for a lot of years now, Russia has seemed to content not to have any important allies and to pursue their agenda more or less alone. The United States has the rest of the West, but Russia and China don't even really get along that well most of the time.
Would anyone that mattered come to Russia's aid if, say, the US invaded them? It seems unlikely.
And it seems they've been this way as long I can think of, since at least Byzantine times. I don't really think Serbia or Syria count as serious allies in the way that Japan and the EU count for the United States, for example.
Can anyone shed some light?
I'd have to disagree. While Russia and China may not have the cozy relationship that the USA enjoys with, say, the UK, they have no serious major disputes, agree on a lot of substantial issues, have several ongoing cooperative agreements (the Shanghai pact security apparatus, for example) and so on. Russia and China collaborate in the UN on a wide range of issues, from security, to counter-terrorism in Asia, to humanitarian intervention (that is, blocking it).
Your "who would come to Russia's aid if the USA invaded them" hypothetical isn't really a good question to ask, since it is such a massively unlikely scenario it tells us very little about contemporary diplomatic arrangements. But I think that if, for some bizarre reason, the USA did invade Russia with the intent of occupying them, China would be extremely likely to intervene in some way. They certainly wouldn't want to see an American occupation of Russia.
But as I say, in measuring alliances, we need to look at more realistic scenarios, and in the same way that the UK and Japan and Australia usually collaborate with the USA on major non-trade issues, China usually collaborates with Russia.
Dmitri Trenin said that the relationship between Moscow and Beijing is "probably the best it's ever been".
Your title seems to be asking whether Russia has always been without major allies, even though the body of your text doesn't. But, at the risk of answering a question you're not actually asking, at several times in its history Russia has been an ally of almost every major European power. France, the UK, Germany (and before that Prussia and Austria) have all had long-lasting treaties with Russia and have fought alongside Russia as an ally in multiple wars.
Source: The Politics of Security in Modern Russia, Mark Galeotti (ed)