US President Barack Obama welcomed 31-new American citizens at a naturalization ceremony and he used the occasion to staunchly defend country's immigration policy. "We should be strong enough to acknowledge, as painful as it may be, that we haven't always lived up to our own ideals. We haven't always lived up to these documents. And the biggest irony of course was is that those who betrayed these values were themselves the children of immigrants. How quickly we forget," said Obama.