I think it will be much more calculated than a full blown war. Voldemort is still vulnerable, even his Death Eaters.... imagine one goes on a power trip and decides he wants to be in charge. Death Eaters are untrustworthy and powerhungry. So in effect, Voldemort needs to conduct the war in a very strategic way to get what he wants but still keep his 'loyal followers' happy at the same time.
as for not having enough witches and wizards for a war.... who said war had to be on a mass scale?
Also, Voldemort is feared all over the world, I'm sure England would have the support of other countries.
And to be quite honest, I'd rather have a giant on my side than to have one against me! They may not be the stealthiest of creatures, but the strength and size is a bonus from the start. Remember in PS/SS - it was said that not many full grown wizards could handle a mountain troll, let alone a few 1st years? Imagine a giant..... very handy to have around I think!
"Now Now Draco... Play nicely"
Harry Potter and the Chamber of Secrets