In the first example, "defend" is in the infinitive form as it is part of a modifying phrase that modifies the noun "Indonesia." You could just as well use the gerund "defending" as the modifier instead.
In the second example, "that" introduces a clause, not a phrase. So "defends" isn't functioning as a modifier but as a verb, a verb whose subject is "Indonesia" and is thus conjugated accordingly.
Maybe it's easier to see if we change out the noun "Indonesia" for a pronoun:
"Seeing him defend himself makes my day."
"Seeing that he defends himself makes my day."
In the first example, "him" begins the direct object of the gerund "seeing," and "defend himself" is a modifier of the pronoun "him." "Him" is not the subject of the verb "defend." "Him" cannot be a subject because it is an object pronoun, not a subject pronoun.
In the second example, "that" begins a clause in which "he" is the subject of the verb "defends." "He" is not an object because "he" is not an object pronoun. "He" is a subject pronoun.
These differences are harder to see in your example because we can use "Indonesia" as either a subject or an object. By switching to a pronoun instead of a noun, we can more readily see that "Indonesia" doesn't have the same function in those sentences, particularly in relation to "defend" or "defends," respectively, and that they themselves don't have the same function, either.