Did God ever really help the Hebrews in battle?

One of the battles in which God's people won against the odds is in 1 Samuel 30. David pursued a raiding party, and ended up battling them for around 24 hours with only 400 men. I'm not sure of the size of the raiding party, but 400 men from the raiding party survived, so I'd assume it was much larger than that.

How about more recent history?

In the Yom Kippur War, they were attacked by a coalition of states led by Egypt and Syria.

The coalition had almost three times the manpower (over 1 million troops against around 400,000 Israeli troops), plus they had anti-air missiles, 20% more armored carriers, 30% more artillery, etc.

Yet, Israel lost only around 2,500 lives compared to around 10-18,000 lives of the enemy. (Not to mention the 2200 tanks lost compared to 400 of the Israelis, 300-500 aircraft lost compared to 100 of the Israelis, etc.)

The coalition got the jump on Israel and attacked when they were not expecting it. It removed their sense of invulnerability (from the Six-Day War and the War of Independence).

Yet, God gave them strength to overpower a force dramatically greater than they were.

Leviticus 26:3-8 shows that if Israel follows God, they will have military victory. This example above, as with all things God related, boils down to faith. God said he'd give them victory and he did.

There are a few other instances of God's direct intervention (as opposed to, say, the Gideon story, which can be attributed to clever tactics).

At Jericho, the priests with the Ark of the Covenant circled the city walls several times, blew their trumpets, and then the walls collapsed (Joshua 6).

Later, in the battle against the Amorites, God rains hailstones down on the enemy army (10:11). Then Joshua asks God to delay the sunset, and he does (10:14, NIV):

There has never been a day like it before or since, a day when the LORD listened to a human being. Surely the LORD was fighting for Israel!

However, God does not grant military victory to Israel all the time. As you note, the nation was defeated many times. An enormous part of the Old Testament is dedicated to understanding why. A key concept is that the people often fall away from God, and he causes military defeat as a punishment or to encourage them to return to faith. God's plan for his people does not necessarily mean that they will win every battle: military victory would be pointless if the people are not faithful. Isaiah speaks about the righteous "remnant" that is left after conquest (eg in 37:31-32). Jeremiah develops the idea in Lamentations. For example, here is 4:12-13 :

The kings of the earth did not believe, nor did any of the peoples of the world, that enemies and foes could enter the gates of Jerusalem. But it happened because of the sins of her prophets and the iniquities of her priests, who shed within her the blood of the righteous.

And yet the people are not completely destroyed (3:22-26),

Because of the LORD's great love we are not consumed, for his compassions never fail. They are new every morning; great is your faithfulness. I say to myself, "The LORD is my portion; therefore I will wait for him." The LORD is good to those whose hope is in him, to the one who seeks him; it is good to wait quietly for the salvation of the LORD.

and there is a chance to return to the way that things should be (3:40)

Let us examine our ways and test them, and let us return to the LORD.

To put a New Testament spin on it: God's victory is in a different category from mere strength of arms, and his kingdom is not the same thing as the Roman/Babylonian/Assyrian/etc. empires. John 18:36 says:

Jesus said, "My kingdom is not of this world. If it were, my servants would fight to prevent my arrest by the Jewish leaders. But now my kingdom is from another place."

The devil tempted Jesus with a vision of temporal power (Matthew 4:8-9 and its parallels) but he refused this kind of dominion.

Here's a list of several events just off the top of my head:

• capture/destruction of Jericho
• Othniel
• Gideon's 300 men against the Midianites
• Samson's final crush of the Philistines
• Elijah and the prophets of Baal

As to why God didn't save them from some oppressors, one of the myriad instances of a single phrase in Judges shows why:

Now the sons of Israel again did evil in the sight of the LORD, so that the LORD gave them into the hands of the {nation} for {number} of years*

^{*_{note "nation" and "number" can be substituted for any oppressor and duration}}